Most failed caulk joints start with the wrong tube, not a clumsy bead. Season 24 put four families on one bench and sorted them by where the joint lives and whether it has to move. One tube for every gap is how a bathroom bead cracks and an exterior joint splits in the first season.
Match the tube to the joint
Latex caulk
Latex caulk (opens in a new tab) is the interior painter's material, the bead where baseboard meets wall. Prime the wall first, then run the bead, so the latex has something to hold. It is paintable. It is the wrong product outdoors and the wrong product in a wet joint.
Acrylic caulk with silicone
Exterior wood and trim move with the weather. Acrylic caulk with silicone (opens in a new tab) is the version the segment used there: acrylic for adhesion across materials, added silicone for water resistance. If the bead cracks in the first season, the joint needed a product that can stretch, not a thicker smear of the same latex.
100% silicone
100% silicone (opens in a new tab) is the wet-area choice, including where a shower meets a tub. It is fully waterproof. You cannot paint it, which is why it is sold in many colors: the color you squeeze is the finished color. A "siliconized" blend is not the same product. Where the joint stays wet, the segment insisted on silicone that is actually silicone.
Butyl sealant
Butyl sealant (opens in a new tab) stays soft for years. That is what you want on metal gutters, on an expansion joint in a tile floor, and in any gap that keeps moving after you leave. It is messy. It sticks to the back of a hand as readily as to metal. Plan the cleanup before you cut the nozzle.
Prep beats a neat bead
- A caulk gun (opens in a new tab) often has a hole in the handle for the nozzle. A pull of the trigger trims the tip at an angle. Cut only as much opening as the joint is wide.
- Pull the bead toward you instead of pushing a pile of sealant ahead of the nozzle. A pulled bead is easier to tool smooth.
- Do not caulk over old silicone. The new material will not stick. The segment's line was that it looks fine for about an hour.
- Scrape the old bead with a razor (opens in a new tab), clean the joint with lacquer thinner (opens in a new tab) or denatured alcohol (opens in a new tab), let it dry, then apply the new sealant.
- For interior latex, primer (opens in a new tab) goes on the wall before the bead.
Leave a joint open when the assembly is supposed to drain. A rain-screen wall needs the bottom of the air gap clear; filling it "to finish the look" turns the gap into a trough. The rain-screen note covers that detail.
DIY confidence
Choosing and running a bead: approachable if you can name the joint first. Interior paint-grade gaps, exterior moving trim, a wet shower joint, and a gutter seam are four different products. Solvents and a razor are the part to slow down for: ventilate, and do not chase a stubborn bead into a finished surface you cannot refinish.
After a tight miter you may not need a bead at all. The layout for that joint is in finish carpentry tools seen on the show.
Where this was shown
The four-tube comparison, the pull stroke, and the "do not caulk over silicone" warning are all in Ask This Old House Season 24, Episode 15.