Exterior caulk is the cheapest weatherproofing you can buy and the one most often bought wrong. A tube costs a few dollars; the gap it fails to seal costs you heat, water damage, or a repaint. And the failure is nearly always predictable from the label — the wrong chemistry in a joint that moves, or a paintable claim taken at face value.
There are four chemistries in the aisle and they are not interchangeable. Getting the choice right takes two questions: how much will this joint move, and does it need paint over it?
The Four Chemistries
Acrylic Latex (and Siliconised Acrylic)
Water-based, easy to tool, cleans up with water, and takes paint perfectly. Plain acrylic latex is cheap and has very limited movement capability — typically rated around ±5% joint movement — which means it cracks in any joint that opens and closes seasonally.
Siliconised acrylic adds silicone to improve flexibility and water resistance while keeping paintability. It is the sensible default for painted exterior trim, and quality varies enormously within the category: look for a stated movement capability and a long warranty rather than the cheapest tube.
Silicone
The most flexible and the most durable against water and UV, with movement capability commonly ±25% or better. It never fully hardens, stays flexible for decades, and is the right product around glass, metal, tile and plumbing fixtures.
Its disqualifying property for much exterior work is that paint will not adhere to it. Not poorly — not at all. Silicone also leaves a residue in the substrate that prevents paint sticking even after the bead is removed, which is why a silicone bead applied where paint later goes creates a permanent problem. Products labelled "paintable silicone" are almost always siliconised acrylics or hybrids rather than true silicone.
Polyurethane
The professional choice for demanding exterior joints. High movement capability, excellent adhesion to masonry, concrete, wood and metal, genuinely paintable, and very durable — 20 years is a realistic expectation on a well-prepared joint.
The trade-offs are handling. It is sticky and difficult to tool, cleans up with solvent rather than water, cures slowly, and typically costs several times an acrylic. For a large expansion joint, a driveway-to-garage joint, or anywhere structural movement is expected, it is worth every bit of the difficulty.
Hybrid Polymers (MS Polymer / SMP)
Modified silane polymers combine most of silicone's flexibility and durability with polyurethane's adhesion and paintability, without the tooling difficulty of polyurethane or the paint incompatibility of silicone. They are the newest category, they cost more than acrylics, and where budget allows they are frequently the best technical answer.
Comparison
| Acrylic Latex | Siliconised Acrylic | Silicone | Polyurethane | Hybrid (MS) | |
|---|---|---|---|---|---|
| Movement capability | ±5% | ±10–15% | ±25–50% | ±25–50% | ±25–50% |
| Paintable | Yes | Yes | No | Yes | Yes |
| Cleanup | Water | Water | Solvent | Solvent | Solvent |
| Tooling ease | Easy | Easy | Moderate | Difficult | Moderate |
| Adhesion to masonry | Fair | Good | Fair | Excellent | Excellent |
| UV resistance | Fair | Good | Excellent | Good | Excellent |
| Typical life | 5 yrs | 10–20 yrs | 20+ yrs | 20+ yrs | 20+ yrs |
| Cost per tube | Lowest | Low | Moderate | High | Highest |
Movement capability is the specification that predicts failure and the one most buyers never look at. A joint between two dissimilar materials — wood trim against brick, for instance — can open and close several percent of its width between summer and winter. Put a ±5% acrylic in a joint that moves 15% and it will split down the middle in its first year, regardless of how carefully it was applied.
What to Use Where
Around Windows and Doors (Painted Trim)
Siliconised acrylic or a hybrid. These joints move with seasonal expansion and get painted, which rules out silicone. Seal the gap between the trim and the siding, and between the trim and the frame — but leave the bottom of the window's exterior trim and any weep holes open, because those are drainage paths and sealing them traps water inside the wall.
Siding Joints and Butt Seams
Siliconised acrylic for painted wood or fibre cement. Check the siding manufacturer's specification, as some fibre cement systems require a specific sealant class and some prefer joints left open with flashing behind.
Wood to Masonry
Polyurethane or hybrid. Two materials with very different thermal movement, and masonry is a demanding substrate for adhesion. This is a joint where the cheap tube reliably fails.
Concrete Expansion Joints and Driveway-to-Garage
Self-levelling polyurethane. Horizontal joints in concrete need a sealant that flows to a flat surface and tolerates substantial movement plus foot and vehicle traffic. A vertical-grade sealant in a horizontal joint leaves a lumpy bead that gets torn out.
Gutters and Metal Flashing
Silicone or a butyl sealant where no paint is required. Metal moves substantially with temperature and these joints see constant water.
Roof Penetrations
A purpose-made roof sealant or flashing cement rather than general-purpose caulk — see our rubberised coating guide for what those products can and cannot do.
Bathrooms and Kitchens (Interior)
Silicone with a mildewcide, in the sanitary grade. Not the same job as exterior caulking but worth noting because the tubes look similar and get confused.
Joint Design — Why Backer Rod Matters
A sealant bead should bond to two surfaces, not three. Bonded to the two sides of a joint, it stretches and compresses freely as the gap moves. Bonded also to the back of the joint, it is restrained in three directions and tears instead of stretching.
Foam backer rod, pushed into the joint before sealing, does two things: it stops three-sided adhesion and it sets the bead depth. The target proportion for a moving joint is roughly half as deep as it is wide, with a minimum depth of about 1/4 inch. A deep, narrow bead is both wasteful and weaker than a correctly shaped one.
For joints too shallow for backer rod, bond-breaker tape at the back of the joint achieves the same thing.
Applying It Properly
1. Remove the Old Sealant Completely
New caulk does not bond to old caulk. Cut it out with a utility knife and a caulk removal tool, and scrape the joint faces clean. If the old material was silicone, expect adhesion problems even after removal — the residue persists, and a silicone digester product is worth using before recaulking.
2. Clean and Dry
Dust, chalking paint, mildew and oil all prevent adhesion. Wire brush loose material, wash, treat mildew, and let the joint dry completely. Most sealants specify a dry substrate, and a damp joint is the most common cause of a bead that peels away within months.
3. Cut the Nozzle Small
Cut at a 45-degree angle, starting smaller than you think you need — you can always enlarge it. An oversized opening lays down far more sealant than the joint needs and makes tooling messy.
4. Push, Do Not Pull
Pushing the gun forces sealant into the joint ahead of the nozzle; pulling drags it across the surface and leaves voids behind. Maintain steady pressure and a consistent speed, and keep the bead continuous — restarting mid-joint leaves a weak point.
5. Tool Within the Skin Time
Tooling presses the sealant against both faces and shapes the bead concave, which is the geometry that handles movement best. Do it within the product's skin time — usually a few minutes — with a wet finger for acrylics, a tooling spatula, or a soapy solution for silicone. After the surface skins, tooling tears it.
6. Respect the Cure Before Painting
Acrylics typically accept paint within an hour or two. Polyurethanes and hybrids often need 24 hours or more. Painting an uncured bead traps solvent and produces a wrinkled finish.
Temperature and Timing
Most exterior sealants specify a minimum application temperature, commonly 40°F, and some acrylics fail outright if they freeze before curing. Beyond the label minimum, there is a subtler reason to think about timing: a joint sealed at its widest is under compression for most of the year, and compression is easier for a sealant to handle than tension.
Practically, that means sealing in moderate weather rather than in a heatwave. A joint caulked on the hottest day of summer is at its narrowest, and the bead spends every winter stretched.
Why Caulk Fails — Five Patterns
Split Down the Middle
The bead has torn in tension because the joint moved further than the sealant's rating allows. This is a chemistry choice problem, not an application one — a ±5% acrylic in a joint that moves 15% will do this no matter how well it went on. Replace with a higher-movement product.
Peeled Away From One Side
Adhesion failure, and nearly always a preparation problem: the joint face was damp, dusty, chalky, oily, or still carried old sealant. Cut it out, clean both faces back to sound material, dry thoroughly, and reapply.
Cracked Across the Back
Three-sided adhesion. The bead bonded to the back of the joint as well as the sides, so it could not stretch and tore internally instead. Backer rod prevents this and is the fix on reapplication.
Wrinkled or Blistered Under Paint
Painted before the sealant cured, trapping solvent that then pushed the paint film. Acrylics usually need an hour or two; polyurethanes and hybrids commonly want 24 hours or more. Check the tube rather than assuming.
Mildew Blackening
Common on acrylics in shaded, damp locations, since the acrylic binder is organic and mildew colonises it. Clean with a dilute bleach solution, and where recurrence is likely choose a product with a stated mildewcide or move to silicone if paint is not required.
How Much to Buy
A standard 10.1-ounce cartridge yields roughly 25 to 30 linear feet of a 1/4-inch bead, dropping to about 12 feet at 3/8 inch and around 7 feet at 1/2 inch. Measure the joints, work out the bead size the gap actually needs, and add a spare tube — running out mid-joint means a cold joint at the restart.
For a typical house exterior, sealing around windows and doors and the main trim joints, most people need somewhere between six and fifteen cartridges. It is worth buying one grade up from the cheapest across that quantity: the price difference over a dozen tubes is small against the labour of doing the job twice.
The Short Version
Buy for movement capability, not for price. A quality siliconised acrylic handles most painted exterior trim, and the difference between a cheap tube and a good one is the difference between recaulking in two years and in fifteen.
Use polyurethane or a hybrid wherever the joint is demanding: wood to masonry, expansion joints, anything horizontal, anything structural.
Use silicone only where no paint will ever go, and understand that the decision is permanent — silicone residue prevents paint adhesion even after the bead is removed.
And use backer rod in anything deep. Two-sided adhesion at half-depth-to-width is what lets a bead move instead of tearing, and it is a few dollars that changes how long the job lasts.
Frequently Asked Questions
It depends on the joint, and the deciding specification is movement capability — how much the gap opens and closes seasonally. For painted exterior trim, windows and siding, a quality siliconised acrylic rated around ±10–15% is the sensible default, and the gap between a cheap tube and a good one is the difference between recaulking in two years and in fifteen. For demanding joints — wood against masonry, concrete expansion joints, anything horizontal — use polyurethane or a modified silane hybrid, both rated ±25–50% with excellent adhesion to masonry. Use silicone only where nothing will ever be painted. Plain acrylic latex, rated around ±5%, cracks in any joint that actually moves.
No, and this is the most consequential thing to know about silicone. Paint does not adhere to cured silicone at all — it beads up and peels. Worse, silicone leaves a residue in the substrate that prevents paint from adhering even after the bead has been cut out, so a silicone bead applied where paint later goes creates a semi-permanent problem that needs a silicone digester product to resolve. Tubes labelled 'paintable silicone' are almost always siliconised acrylics or hybrid polymers rather than true silicone; read the chemistry on the back rather than the claim on the front. Where a joint needs both flexibility and paint, use a siliconised acrylic, a polyurethane, or an MS polymer hybrid.
Foam rod pushed into a joint before sealing, and yes for anything deep. It does two things. It prevents the sealant bonding to the back of the joint: a bead bonded only to the two sides can stretch and compress freely as the gap moves, while one also bonded at the back is restrained in three directions and tears rather than stretching. And it sets the bead depth — the target for a moving joint is roughly half as deep as it is wide, with a minimum of about 1/4 inch, because a deep narrow bead is both wasteful and weaker than a correctly proportioned one. For joints too shallow for rod, bond-breaker tape at the back achieves the same thing.
Anything that is a drainage path by design. Weep holes in brick veneer let water that has entered the wall cavity escape; sealing them traps it inside. The bottom edges of horizontal siding boards are drainage and ventilation gaps. Window weep slots along the bottom of the frame drain the sill. The gap under a door threshold sheds water. Caulking any of these stops a small draught and causes a much larger moisture problem inside the wall assembly, where it will not be visible until there is rot. The general principle: seal the top and sides of things, leave the bottom open so water that gets in can get back out.
Five years for plain acrylic latex, ten to twenty for a good siliconised acrylic, and twenty or more for silicone, polyurethane and hybrid polymers — but only if the chemistry matches the joint's movement. A ±5% acrylic in a joint that moves 15% splits within its first year no matter how carefully it was applied, which is why movement capability is the specification that predicts failure. Preparation matters nearly as much: new caulk does not bond to old caulk, so the previous bead has to be cut out completely, and the joint faces cleaned and dried. A damp joint at application is the most common cause of a bead peeling away within months.
Most exterior sealants specify a minimum around 40°F, and some water-based acrylics fail permanently if they freeze before curing, so check the tube. But there is a subtler timing consideration than the label minimum. A joint is at its narrowest in summer heat and widest in winter cold, and sealant handles compression better than tension — so a bead applied on the hottest day of the year spends every subsequent winter stretched to its limit. Sealing in moderate weather, spring or autumn, puts the bead nearer the middle of its range and meaningfully extends its life. Also allow the joint to be genuinely dry, which after rain can take longer than the surface suggests.
Push. Pushing forces sealant into the joint ahead of the nozzle, filling it properly; pulling drags the bead across the surface and leaves voids behind that you cannot see until the joint fails. Cut the nozzle at a 45-degree angle and start smaller than you think you need, since an oversized opening lays down more material than the joint requires and makes tooling messy. Keep steady pressure and a consistent speed, and avoid stopping mid-joint, because each restart is a weak point. Then tool the bead within the product's skin time, usually a few minutes, pressing it against both faces and shaping it concave — that geometry handles movement best. Tooling after the surface has skinned tears it.
In everyday use the words are interchangeable, and manufacturers use both on similar products. Where a distinction is drawn, 'caulk' tends to mean the lower-movement, paintable, easily tooled materials — acrylic latex and siliconised acrylic — used to close gaps in trim and siding, while 'sealant' implies higher movement capability and a more demanding application, such as silicone, polyurethane and hybrid polymers used in structural and expansion joints. Rather than relying on the name, read two things on the label: the stated movement capability as a percentage, and whether it is paintable. Those two facts determine whether the product suits your joint far more reliably than which of the two words is printed on the tube.