Overview
Caulk and sealant do one job: fill and seal gaps and joints against air and water. Simple enough. What trips people up is that the type matters far more than the act of squeezing a bead. Pick wrong and it fails in a season, or it cures into something paint will not stick to, and now you are chasing your own work. A handful of properties decide everything. Is it paintable or not? How much movement can it take before it cracks? Is it rated for interior only, or for exterior sun and UV? Does it resist water and mold in wet areas? And will it actually grab the substrate you are sticking it to? Get those five right and the rest is tooling. The families you will reach for are acrylic latex (painter's caulk), silicone, polyurethane, hybrid MS polymer, and butyl. Each owns a lane. Acrylic for paintable interior trim, silicone for wet and unpainted, poly and hybrid for exterior and movement, butyl for metal and flashing laps. One thing to set straight up front: caulk is not a substitute for flashing, and it is not a fix for a structural gap. If a joint should have been flashed or framed correctly, sealant only hides the problem until it leaks.
Acrylic Latex (Painter's Caulk)
This is the water-based interior standard, and it earns the spot. Acrylic latex is cheap, easy to gun, smooths with a wet finger, and cleans up with water before it sets. The reason it rules interior trim is simple: it takes paint perfectly. You bead it into the gap where baseboard meets wall, where crown meets ceiling, around door and window casing, then paint right over it and the line disappears. No other family does that as cleanly. If you are running interior trim, this is the tube on your bag. The trade-off is movement. Standard acrylic has low flexibility, so on a joint that expands and contracts a lot it cracks and pulls away. It is not built for wet areas, big exterior gaps, or anywhere it sits in standing water. Exterior life is shorter than the tougher families. Siliconized acrylic, sometimes sold as acrylic with silicone added, buys you a little more flex and water resistance while staying paintable, which makes it a reasonable step up for trim near a tub surround or a kitchen backsplash. But for the interior paintable work it was made for, plain painter's caulk is hard to beat. See interior trim types for where these joints actually live.
Silicone
Silicone is the waterproof, flexible pick, and it is the one I want anywhere water lives. It shrugs off water, resists mold and mildew, handles temperature swings, and stays flexible for decades instead of going brittle. That is why it is the standard around kitchens, baths, showers, tubs, sinks, and glass. Seal the joint where a tub meets tile, or where a glass shower panel meets the pan, and silicone holds while everything around it moves and gets soaked daily. The catch, and it is a big one, is that most silicone is not paintable. Paint will not bond to it; it beads up and peels. So silicone is the wrong call anywhere you intend to paint over the bead, which rules it out for most interior trim. It is also harder to tool than acrylic, it does not clean up with water, it smells like vinegar while it cures, and once it is cured almost nothing sticks to it, including fresh silicone, so you cannot just bead over an old failed joint. Pull the old stuff first. For wet, non-painted joints, pure silicone is the answer. Just keep it out of the places you plan to paint.
Polyurethane and Hybrid (MS Polymer)
Polyurethane is the tough exterior and structural workhorse. It is paintable, it grabs hard, and it flexes, which makes it the pick for exterior joints, expansion joints, sidewalks and concrete control joints, masonry, and anywhere a joint carries load or moves with the building. It is durable and outlasts acrylic outdoors by a wide margin. The downsides are real: it is messy to work, it does not clean up with water, it cures slowly, and over years of direct sun the surface degrades from UV unless it is painted or formulated for it. Then there is hybrid, sold as MS polymer or modified silane, and it is the modern do-it-all. It paints like polyurethane and seals like silicone: waterproof, flexible, UV-stable, low odor, and it bonds to almost anything wet or dry without primer, including damp surfaces and a long list of substrates that fight other sealants. It is the premium tube and it costs more than the rest, but it covers the widest range of jobs of anything on the shelf. If you want one product that handles exterior siding joints, structural movement, and a paintable finish, hybrid is it. It pairs naturally with flashing details and window perimeters, both worth reading up on separately.
Butyl, and How to Choose
Butyl rubber is the sticky, aggressive waterproofer. It grabs metal and stays grabbed, which is why it lives on gutters, gutter seams, flashing laps, metal-to-metal joints, and below-grade work where you need adhesion and water resistance over looks. It is not pretty, it is not paintable, and it strings and smears while you tool it, so it goes where nobody sees it. Now the decision rule, because this is the whole game. Painting over it on interior trim? Acrylic latex. Wet and not painted, like a bath or kitchen? Silicone. Exterior, structural, moving, and needs to take paint? Polyurethane or an MS-polymer hybrid. Gutters, flashing laps, and metal? Butyl or hybrid. Match the sealant to three things: paintability, movement, and wet or UV exposure. Get those right and you are most of the way there. The other half is prep, which matters as much as the tube. Clean and dry the joint, push backer rod into deep gaps so the bead is the right shape and not bonded on three sides, and tool it so it actually contacts both faces. And the rule worth repeating: caulk is not flashing. Do not caulk what should have been flashed or built right.
FAQ
Can I paint over silicone? No, and do not try. Paint will not bond to most silicone; it beads up and peels. If the bead needs paint, use acrylic latex or an MS-polymer hybrid instead. The only fix on a cured silicone joint is to cut it out and replace it with a paintable sealant.
What is the difference between caulk and sealant? In practice the words get used interchangeably, but the rough split is that caulk usually means the cheaper, lower-movement, paintable stuff like acrylic latex, while sealant means the high-performance flexible families like silicone, polyurethane, and hybrid. What matters is the properties on the label, not the word on the tube.
Why does my caulk keep cracking? Almost always wrong product for the movement, or bad joint prep. Low-flex acrylic on a joint that expands and contracts will split. Fix it with a flexible sealant (hybrid or polyurethane outside, silicone in wet areas), and use backer rod in deep joints so the bead can stretch instead of tearing.
What is backer rod and do I need it? It is a foam cord you push into a deep joint before sealing. It controls bead depth, saves material, and lets the sealant bond to only the two faces of the joint so it can flex without tearing. For any deep or wide gap, yes, use it - it is the difference between a bead that lasts and one that fails early.