The best concrete crack filler for most homeowners is a self-leveling polyurethane sealant such as Sikaflex Self-Leveling Sealant, because most of the cracks people want to fix are horizontal cracks and joints in driveways, sidewalks and patios, and those cracks move. Concrete expands in summer, shrinks in winter and heaves a little with frost, so a crack filler on a slab has to stretch and recover rather than set hard. A rigid patch in a moving crack simply cracks again along one edge within a season.
The exceptions matter, though. A crack in a poured basement wall that you want to bond shut needs an epoxy injection kit. A crack with water actively running through it needs hydraulic cement, which sets in minutes against the flow. A broad, shallow spall on a step or slab edge needs a vinyl patching compound, not a sealant. And a crack that is wide, offset or growing is not a product problem at all — it is a reason to call a structural engineer before you fill anything.
One point of confusion worth clearing up straight away: concrete crack fillers and asphalt crack fillers are not interchangeable. Asphalt fillers are rubberized asphalt emulsions or hot-pour compounds designed to bond to a bitumen-based surface; on concrete they bond poorly, stay soft and leave a black stripe across a grey slab. If your driveway is blacktop, our guide to the best asphalt crack filler covers that chemistry. Everything below is for concrete.
Our Top Picks
- BEST OVERALL: Sikaflex Self-Leveling Sealant
↓ Jump to Review - BEST FOR APPEARANCE: Sashco Slab Concrete Crack Repair
↓ Jump to Review - BEST FOR VERTICAL CRACKS: Sikaflex-1a Polyurethane Sealant
↓ Jump to Review - BEST FOR BASEMENT WALL CRACKS: Simpson Strong-Tie Crack-Pac Injection Epoxy
↓ Jump to Review - BEST FOR ACTIVE LEAKS: UGL Drylok Fast Plug
↓ Jump to Review - BEST FOR SPALLS & CHIPPED EDGES: QUIKRETE Vinyl Concrete Patcher
↓ Jump to Review
Quick Comparison
| Product | Type | Best for | Flexible? |
|---|---|---|---|
| Sikaflex Self-Leveling Sealant | Self-leveling polyurethane | Driveway, sidewalk and patio cracks and joints | Yes |
| Sashco Slab Concrete Crack Repair | Self-leveling water-based sealant | Visible slab cracks where appearance matters | Yes |
| Sikaflex-1a Polyurethane Sealant | Non-sag polyurethane caulk | Vertical cracks, steps, foundation exterior, wall-to-slab joints | Yes |
| Simpson Strong-Tie Crack-Pac Injection Epoxy | Two-part injection epoxy | Dry, non-moving cracks in poured basement walls | No (rigid bond) |
| UGL Drylok Fast Plug | Hydraulic cement | Cracks and holes with active water leaks | No |
| QUIKRETE Vinyl Concrete Patcher | Polymer-modified patching compound | Spalls, chipped edges, shallow surface damage | No |
The Best Concrete Crack Fillers
These six cover essentially every crack a homeowner will meet. They are not six versions of the same thing ranked against each other; they are the strongest option in each job category, ranked by how many people need that job done.
What We Like
- Self-levels to a neat, flat bead with no tooling
- Polyurethane flexibility suits moving slab cracks and joints
- Grey colour sits reasonably close to most concrete
- Fits a standard caulk gun
What We Don’t Like
- Horizontal surfaces only
- Needs backer rod in wider cracks to keep usage sensible
- Messy if it gets on surrounding concrete; mineral spirits cleanup
Product Specs
- Type: Self-Leveling Polyurethane Sealant
- Rank in this guide: #1
Sikaflex Self-Leveling Sealant is a one-part, moisture-curing polyurethane designed specifically for horizontal joints and cracks in concrete — driveways, sidewalks, patios, garage slabs and the control joints that are cut into them. It comes in a standard cartridge, flows out of the nozzle like thick syrup, and settles into a flat, even bead without tooling. That self-leveling behaviour is the whole point: on a slab you want the sealant to find its own level just below the surface, where tyres and shovels pass over it instead of catching an edge.
Polyurethane is the right chemistry for this job because it bonds well to clean concrete, stays elastic through freeze-thaw cycles, and tolerates the seasonal opening and closing that tears rigid patches apart. It is also a known quantity among contractors — Sika is one of the most widely specified sealant makers in commercial concrete work, and this is the homeowner-sized version of the same idea.
The trade-offs are the usual polyurethane ones. It is horizontal-only, so it will run straight out of a crack on a step riser or wall. It needs a clean, dry crack and backer rod in anything wider than about 1/4", or you will pour cartridge after cartridge into the void. And like any moisture-curing urethane, it needs several days in cool or dry weather before it reaches full cure, so it should go down before you apply a concrete sealer, not after.
What We Like
- Textured finish blends with concrete far better than smooth sealants
- Water cleanup and low odour
- Flexible after cure for moving cracks
- Self-leveling, no tooling needed
What We Don’t Like
- Needs dry weather during the first cure
- Horizontal surfaces only
- Less robust than polyurethane in heavy vehicle traffic
Product Specs
- Type: Self-Leveling Water-Based Sealant
- Rank in this guide: #2
Sashco Slab is the pick for cracks you look at every day — a front walk, a patio by the back door, a pool deck — where a glossy grey line would bother you. It is a flexible, self-leveling sealant for horizontal concrete, and Sashco gives it a textured, sandy finish intended to blend with the surface of a slab rather than sit on it like a strip of rubber. From a few feet away a well-filled crack reads as a hairline instead of a stripe.
It is water-based, which makes it the friendlier product to work with: it cleans up with water while wet, has far less odour than a solvent-bearing urethane, and is more forgiving if a bit lands where it should not. It stays flexible after cure, so it handles the seasonal movement of an exterior slab in the same way the Sikaflex does.
The trade-off is weather sensitivity during application. A water-based sealant needs a dry forecast and mild temperatures for its first day or so; rain on a fresh bead will wash it out. On a heavily trafficked driveway joint that takes vehicle loads every day, a polyurethane is the more robust choice. On walks and patios, Slab is the better-looking one.
What We Like
- Non-sag: works on vertical and overhead cracks
- Designed for moving joints in concrete and masonry
- Paintable once cured, several colours available
- Ideal for slab-to-foundation joints
What We Don’t Like
- Must be tooled quickly; short working time
- Stiff to dispense in cold weather
- Uncured material is hard to clean off
Product Specs
- Type: Non-Sag Polyurethane Caulk
- Rank in this guide: #3
Self-leveling sealants are useless on anything that is not flat. For a crack in a step riser, a retaining wall, the outside face of a foundation, or the joint where a slab meets a house wall, you need a non-sag polyurethane caulk that stays exactly where you tool it. Sikaflex-1a is the long-standing contractor standard in that category: a one-part, moisture-curing polyurethane built for expansion joints, control joints and cracks in concrete and masonry.
It is thick enough to hold a bead overhead, bonds strongly to clean concrete, and is designed for joints that move — which is why it is a staple for the slab-to-foundation joint, where the driveway or patio shifts independently of the house. It comes in several colours, including greys that sit close to most concrete, and it can be painted once cured if you need to match a coated wall.
The trade-off is working time and cleanup. It is stiff to gun in cold weather, it has to be tooled within minutes, and uncured urethane is stubborn on hands and tools. It also does not self-level, so on a horizontal crack you will get a smoother result faster with the Sikaflex Self-Leveling product. For the broader range of exterior joints around windows, trim and siding, see our guide to the best exterior caulk.
Simpson Strong-Tie Crack-Pac Injection Epoxy
See It on AmazonWhat We Like
- Fills the crack through the full wall thickness, not just the surface
- Rigid bond restores continuity in stable cracks
- Kit format mirrors professional injection technique
- A fraction of the cost of a contractor injection
What We Don’t Like
- Rigid: not for cracks that are still moving
- Needs a dry crack; not for active leaks
- Poured walls only, not block
- Paste-over must be ground off afterwards if appearance matters
Product Specs
- Type: Epoxy Injection Kit
- Rank in this guide: #4
A vertical or diagonal shrinkage crack in a poured concrete basement wall is the classic DIY injection job, and Simpson Strong-Tie's Crack-Pac is one of the most widely available homeowner-scale systems. It is a low-viscosity, two-part epoxy that dispenses through a mixing nozzle from a cartridge, and the kit approach is the same as a professional injection: you bond plastic injection ports along the crack, seal the face of the crack between them with a paste-over epoxy, then inject from the lowest port upward until resin appears at the next one.
The advantage of epoxy over any surface sealant is depth. Injected under pressure, it fills the crack through the full thickness of the wall and cures into a bond that is typically stronger than the surrounding concrete. That makes it the right choice for a dry, stable crack where you want the wall to behave as one piece again and to stop the seepage that comes and goes with wet weather.
The limitations are important. Epoxy is rigid, so it suits cracks that have stopped moving; if a crack is still opening, the wall will simply crack again beside the repair. It needs a dry crack to bond, so a wall that is actively leaking is better served by hydraulic cement first or a flexible polyurethane foam injection. And it is for poured concrete only — block walls are hollow and cannot be injected this way. Injection fixes the crack; it does not fix the conditions behind a damp basement, such as grading and gutters.
What We Like
- Sets in minutes, even with water flowing
- Expands as it cures to lock into the opening
- Works on poured concrete and block
- Inexpensive and widely stocked
What We Don’t Like
- Rigid; will crack again if the crack moves
- Very short working time — mix small batches
- Requires chiselling out a keyway for best results
Product Specs
- Type: Hydraulic Cement
- Rank in this guide: #5
When water is visibly seeping or running through a crack or tie-rod hole in a basement wall, nothing that needs a dry surface will work: the water pushes sealants out before they cure. Hydraulic cement is the answer, and UGL Drylok Fast Plug is the version most hardware stores keep on the shelf. It is a cement-based powder that you mix to a stiff putty, press into the crack, and hold in place while it sets in roughly three to five minutes — even against flowing water.
It also expands slightly as it sets, which locks it into the opening rather than leaving a shrinkage gap at the edges. The technique is to cut the crack into a keyway that is wider at the back than at the face, so the plug cannot be pushed out, then mix only as much as you can place in a couple of minutes.
The trade-off is that hydraulic cement is rigid and is a plug rather than a crack sealant. It stops a leak at a specific point; it does not flex with a moving crack and it does not seal a long hairline. Treat it as the first step — stop the water, let it dry, then follow with injection on the crack or a masonry waterproofer on the wall.
What We Like
- Feathers to a thin edge on worn and spalled surfaces
- Looks like concrete once cured
- Rebuilds step nosings and broken slab edges
- Mixes with water only
What We Don’t Like
- Rigid: wrong for moving cracks
- Deep repairs need multiple lifts
- Needs damp-curing in hot, dry weather
Product Specs
- Type: Vinyl Patching Compound
- Rank in this guide: #6
Some concrete damage is not a crack at all. Spalled patches where the surface has flaked off, chipped step nosings, pitted areas around a crack and the broken edge of a slab all need a material that can be troweled flat and built back to shape. QUIKRETE Vinyl Concrete Patcher is a polymer-modified cement powder you mix with water; the vinyl content is what lets it bond to old concrete and be feathered out to a thin edge, which plain sand-mix concrete cannot do.
It finishes grey and cement-textured, so a patch reads as concrete rather than as a sealant. It is the product for the wide, shallow damage that would swallow a whole cartridge of sealant and still look wrong, and it can be used to rebuild the shoulders of a badly broken crack before the crack itself is sealed with something flexible.
The trade-off is rigidity. A cementitious patch will not move with a working crack, so do not use it to fill a crack that opens and closes — it will debond along one edge. Deep holes should be built up in layers as the bag directs, and in hot or windy weather the patch needs to be kept damp so it does not dry before it cures.
Read the Crack Before You Buy
Almost every failed crack repair comes from matching the product to the surface (driveway, basement) instead of to the behaviour of the crack. Spend five minutes looking at it first. There are four kinds that matter.
Hairline Shrinkage Cracks
Thin, random cracks under about 1/16" wide, often in a spider-web or map pattern, that appeared within the first year or two. Concrete shrinks as it cures and almost every slab develops some. They are cosmetic, rarely move much, and are frankly too narrow for most fillers to enter — a self-leveling sealant will bridge over the top and peel. The best treatment for a slab covered in hairlines is a good penetrating sealer, which wicks into them and limits water entry; our guide on how to seal a concrete driveway walks through it. If one hairline bothers you, widen it to about 1/4" with an angle grinder and crack-chasing blade so a sealant has something to grip.
Moving Cracks and Expansion Joints
Cracks between about 1/8" and 1" in slabs that get noticeably wider in winter and narrower in summer, plus the sawcut control joints and expansion joints between slab sections. This is the largest category and where flexible sealants earn their place. Rigid fillers — vinyl patch, mortar, epoxy — fail here every time, because the crack opens and the repair cannot follow it.
Horizontal Surface Joints
The joint between a driveway and a garage slab, a walkway and a step, or a patio and the house foundation. These separate two pieces of concrete that move independently, often by more than a random crack does, and they are where water gets under slabs and into basements. They take a flexible sealant over backer rod, and the slab-to-house joint usually needs a non-sag product because one face is vertical.
Structural Cracks
Cracks that are wide, offset (one side higher than the other), horizontal in a basement wall, stair-stepping through a block wall with displacement, or visibly growing. These are symptoms of settlement, soil pressure or a failing footing. Filling them hides the evidence without addressing the cause, and the next section covers what to do instead.
Filler Types Explained
| Filler type | Typical width | Flex | Use it for | Avoid it for |
|---|---|---|---|---|
| Self-leveling polyurethane | 1/8" – 1" (backer rod over 1/4") | High | Driveways, walks, patios, control joints | Vertical surfaces |
| Self-leveling silicone | 1/8" – 1"+ | Very high | Wide joints with large movement, full sun | Anything you plan to seal or paint over |
| Non-sag polyurethane caulk | 1/8" – 1/2" typical | High | Steps, walls, slab-to-house joints | Large flat cracks (hard to level) |
| Epoxy injection | Hairline – 1/4" | None | Dry, stable cracks in poured walls | Moving or leaking cracks, block walls |
| Polyurethane foam injection | Hairline – 1/4" | Moderate | Leaking or slightly moving poured-wall cracks | Cracks you want structurally bonded |
| Hydraulic cement | Holes and cracks, keyed out | None | Active water leaks | Moving cracks, cosmetic slab repair |
| Vinyl patching compound | Spalls, chips, featheredge | None | Surface damage, broken edges | Any crack that moves |
Self-leveling polyurethane is the workhorse for flat exterior concrete. It bonds well, flexes, and can usually be sealed over once fully cured, which matters if you seal your driveway every few years.
Self-leveling silicone sealants handle even more movement and shrug off UV indefinitely, which is why highway departments use them in pavement joints. The catch for homeowners is that nothing sticks to cured silicone — not sealer, not paint, not a later polyurethane repair. If the slab will be sealed or the joint might ever be redone with another product, polyurethane is the easier long-term choice.
Non-sag polyurethane caulk is the same chemistry thickened so it holds its shape. Use it anywhere gravity would pull a self-leveling product out of the crack.
Epoxy and polyurethane injection are basement-wall techniques. Epoxy bonds a dry, stable crack and restores the wall's continuity; polyurethane foam reacts with water, expands to fill the crack, and stays slightly flexible, which makes it the better choice for a crack that leaks or still moves a little. Neither belongs on a driveway.
Hydraulic cement is for stopping water, not for sealing cracks. Vinyl patching compound is for rebuilding surfaces, not for filling cracks that move. Both are rigid, and both fail when asked to do a flexible sealant's job.
When a Crack Is Structural
No crack filler on this page fixes a structural problem, and some of them make it worse by hiding the evidence an engineer needs to see. Before you fill a crack in a foundation wall, basement floor or attached slab, check for these warning signs:
- Horizontal cracks in a basement wall, especially near mid-height. These usually mean soil pressure is pushing the wall inward, and they are the most serious crack a homeowner is likely to find.
- Any bowing or leaning. Hold a long level or string line against the wall. A wall that is no longer flat is moving.
- Offset across the crack. If one side sits higher or further out than the other, the concrete has displaced, not just cracked.
- Stair-step cracks in block walls with displaced blocks or widening mortar joints.
- Width over about 1/4" in a foundation wall, or any crack that is getting wider.
- Matching symptoms upstairs — doors and windows that suddenly stick, drywall cracks at door corners, or floors that slope.
To find out whether a crack is active, mark both ends with a pencil and the date, and measure its width at two or three marked points with a ruler or a cheap plastic crack gauge. Check again every month or two through a full season. A crack that has not changed in width or length is probably a settled shrinkage crack; one that grows is telling you something.
If you see any of the warning signs, call an independent structural engineer before calling a foundation repair company. An engineer's inspection typically runs a few hundred dollars and gives you an unbiased diagnosis, whereas a repair contractor's free inspection tends to conclude that you need the repair they sell. Once the cause is dealt with, the crack itself can be injected or sealed as part of the fix. For more on what water through a foundation means, see our guide to waterproofing basement walls.
Backer Rod and Joint Depth
Backer rod is the most overlooked part of a crack repair and the one that most affects whether it lasts. It is a round foam cord, usually closed-cell polyethylene, that you press into a crack before the sealant goes in. Use it in any crack wider than about 1/4", and in any crack deeper than about 1/2".
It does three jobs. First, it controls sealant depth. A flexible sealant works best as a thin, wide band: the common rule is a depth about half the width for joints wider than 1/2", and roughly equal to the width in narrower ones, with a minimum of about 1/4" and a maximum of about 1/2". A sealant bead that is too deep cannot stretch properly and tears in the middle when the crack opens.
Second, it prevents three-sided adhesion. If sealant bonds to both walls of the crack and to the bottom as well, it cannot stretch freely, and it tears away from one side. Backer rod gives the sealant a non-stick floor so it bonds only to the two sides that move apart.
Third, it saves a great deal of material. A 1/2" crack that runs four inches deep through a slab will swallow many cartridges of sealant without it. With backer rod set at 1/4"–1/2" below the surface, one cartridge goes a long way.
Sizing: choose rod about 25% wider than the crack so it stays put by compression — 3/8" rod for a 1/4"–5/16" crack, 5/8" rod for a 1/2" crack, and so on. Push it in with a blunt tool such as a putty knife or a wooden paint stick, not a screwdriver; puncturing closed-cell rod lets gas escape into the curing sealant and leaves bubbles. For very deep or irregular cracks, pour dry sand in first to within an inch of the surface, then set the rod on top.
How to Fill a Concrete Crack
1. Check the Weather
Most sealants want surface temperatures between roughly 40°F and 90°F, a dry crack, and no rain for at least a day — check the cartridge for the exact range. Cool mornings in spring or fall are ideal for slabs, because the crack is near its average width rather than fully closed by summer heat.
2. Open and Clean the Crack
This is where repairs succeed or fail. Pull out old sealant, weeds and loose fragments with a screwdriver or a crack-cleaning wire wheel. If the edges are crumbly, chip back to sound concrete. Narrow cracks that you want to fill should be widened to at least 1/4" with a grinder and crack-chasing blade. Then blow the crack out completely with a leaf blower or compressed air, or vacuum it. Sealant bonds to clean, sound, dry concrete, and to nothing else.
3. Let It Dry
If you rinsed the crack or it rained recently, wait until it is dry to the bottom — usually a full day of dry weather. Polyurethane sealants applied to damp concrete can bubble and lose adhesion.
4. Set the Backer Rod
Press rod into any crack over 1/4", aiming for a sealant depth of 1/4"–1/2" below the surface as described above.
5. Apply the Sealant
Cut the cartridge nozzle to slightly smaller than the crack width. For self-leveling products, fill from the bottom of the crack upward in one continuous pass and stop slightly below the surface — about 1/8" — so the cured bead sits recessed where tyres and shovels pass over it. For non-sag caulk, fill the crack and tool the bead within a few minutes with a spoon or caulk tool, pressing it against both sides. Mask the edges with tape on decorative concrete.
6. Let It Cure
Cure times vary with product, depth, temperature and humidity, so the label is the authority. As a rough guide, self-leveling polyurethanes skin over within hours and are usually fine for foot traffic within a day or so, with vehicles kept off for a few days and full cure taking a week or more in cool or dry weather. Water-based sealants dry faster on the surface but are more vulnerable to rain early on. Hydraulic cement sets in minutes; injection epoxy cures over roughly a day before the ports and paste-over can be removed.
7. Seal the Slab Afterwards
Once the crack repairs are fully cured, a penetrating or film-forming sealer over the whole slab reduces water entry everywhere else. Our comparison of the best concrete sealers covers which type suits driveways, patios and garages.
Mistakes That Make Repairs Fail
Using a rigid patch in a moving crack. The single most common failure. Mortar, vinyl patch and epoxy look perfect for a few months, then crack along one edge at the first cold snap.
Skipping the cleanout. Sealant on top of dust, old sealant or a film of dirt bonds to the dirt. Ten minutes with a wire brush and blower decides whether the repair lasts a season or many years.
Overfilling. A bead proud of the surface gets caught by tyres, shovels and snowplough blades and torn out. Stop just below flush.
Leaving out backer rod. Too-deep sealant tears in the middle, sealant that bonds to the bottom of the crack tears from the sides, and both waste material.
Using silicone on a slab you plan to seal. Sealers and paints will not bond to it, and it leaves a visible stripe that cannot be coated.
Using asphalt filler on concrete. It is the wrong chemistry and the wrong colour, and it stays soft and tacky in summer.
Filling a structural crack and moving on. If a wall is bowing or a crack is growing, a tidy repair only hides the problem until the next one appears.
The Short Version
For the cracks most people are dealing with — in driveways, sidewalks and patios — buy a self-leveling polyurethane such as Sikaflex Self-Leveling Sealant, a bag of closed-cell backer rod sized to the crack, and spend more time cleaning the crack than filling it. Choose Sashco Slab instead where the crack is in plain sight and a texture that blends matters more than vehicle loads.
Use Sikaflex-1a for anything vertical and for the joint between slab and house. Use epoxy injection for a dry, stable crack in a poured basement wall, Drylok Fast Plug to stop a crack that is actively leaking, and vinyl concrete patcher for spalls and chipped edges rather than cracks.
And if the crack is horizontal in a basement wall, offset, wider than about 1/4" or still growing, put the caulk gun down and call a structural engineer first.
Frequently Asked Questions
A self-leveling polyurethane sealant such as Sikaflex Self-Leveling Sealant, used with closed-cell backer rod in any crack wider than about 1/4". Driveway cracks open and close with temperature and frost, so the filler has to stay flexible; rigid products such as mortar, vinyl patch or epoxy crack along one edge within a season. The self-leveling formula flows into a flat bead without tooling, and you should stop about 1/8" below the surface so tyres and snow shovels pass over the repair. Clean the crack thoroughly and let it dry before filling — preparation decides how long the repair lasts far more than the brand does.
Yes for any crack wider than about 1/4" or deeper than about 1/2". Backer rod controls the depth of the sealant, which should be roughly half the width in wider joints and no more than about 1/2" deep, so the bead can stretch without tearing. It also stops the sealant bonding to the bottom of the crack, which would leave it unable to move freely, and it saves a great deal of material in deep cracks. Choose rod about 25% wider than the crack, press it in with a blunt tool rather than anything sharp, and for very deep cracks fill most of the depth with dry sand first.
No. Asphalt crack fillers are rubberized asphalt emulsions or hot-pour compounds made to bond with bitumen-based blacktop. On concrete they bond poorly, stay soft and tacky in summer heat, and leave a black stripe across a grey slab. Concrete needs a polyurethane, silicone or water-based concrete sealant instead. The reverse is also true: concrete sealants are not the economical choice for long cracks in an asphalt driveway, where purpose-made asphalt fillers are cheaper per foot and blend in.
Warning signs include horizontal cracks in a basement wall, any bowing or leaning in a wall, one side of the crack sitting higher or further out than the other, stair-step cracks with displaced blocks, foundation cracks wider than about 1/4", and cracks that are getting longer or wider. Symptoms upstairs, such as doors that suddenly stick or drywall cracking at door corners, add to the concern. Mark the ends of the crack and measure its width at a few points, then recheck every month or two. If any warning sign is present, have an independent structural engineer assess it before filling anything or hiring a repair company.
If water is actively running through it, stop the flow first with hydraulic cement such as UGL Drylok Fast Plug: chisel the crack into a keyway wider at the back than the face, press in the mixed cement and hold it for the few minutes it takes to set. For a crack in a poured wall that leaks only during wet weather, a polyurethane foam injection kit is the better permanent fix, because the foam expands to fill the crack and stays slightly flexible; epoxy injection suits dry, stable cracks. Either way, deal with the source of the water — grading, gutters and downspouts — or the next weak point will leak instead.
It depends on the product, bead depth, temperature and humidity, so check the label. Self-leveling polyurethane sealants typically skin over within hours and tolerate foot traffic after about a day, but vehicles should stay off for several days and full cure can take a week or more in cool or dry weather because they cure by drawing moisture from the air. Water-based sealants dry faster on top but need a dry forecast for the first day. Hydraulic cement sets in minutes, and injection epoxies generally cure within about a day. Do not apply a slab sealer over crack repairs until they are fully cured.
Usually not with a crack filler. Shrinkage cracks under about 1/16" wide are cosmetic and too narrow for most sealants to enter; a self-leveling product will just sit on top and peel. A penetrating concrete sealer applied to the whole slab is the better treatment, because it wicks into hairlines and limits the water that causes freeze-thaw damage. If a single hairline bothers you, widen it to about 1/4" with a grinder and crack-chasing blade so a flexible sealant has enough depth and sidewall to bond to.
Yes, and it is one of the most worthwhile repairs on the property, because that joint lets water under both slabs. Clean out the old filler, debris and weeds, set closed-cell backer rod about 1/4"–1/2" below the surface, and fill with a self-leveling polyurethane sealant. Where the joint meets a vertical face, such as a slab against the house foundation, use a non-sag polyurethane caulk like Sikaflex-1a instead so it does not run. Avoid rigid fillers here; the two slabs move independently and will crack any mortar or patching compound.