The Old Tradesman

The tips library

Caulk that holds

A bead of caulk is a rubber band glued to two sides of a gap. Most failures I've seen came from what was under it or what it was stuck to, so here's the prep, the depth and the tube to pick, from the sealant makers' own sheets.

Tip 1

Every bit of the old caulk comes out

The trick

Score both edges of the old bead with a utility knife, lift the bulk out with a removal tool, then scrape what's left with a putty knife or razor scraper. Caulk remover on the last of it, let it work, then scrape again.

Why. GE's own how-to: fresh sealant will not bond properly to existing material. New over old lets go the day the old bead does.

The tell. Run a fingernail along the cleaned joint. Any rubbery film left is what your new bead will peel off with.

Old bead hard as a rock? GE softens it with gentle heat from a hair dryer before it comes out. Family Handyman slices it along the walls, squirts caulk remover on what's left and scrapes it with a razor scraper.

When to stop. Tile loose, or the wall soft behind the joint once it's open? Caulk won't fix that. The tile or the backer gets dealt with first.

Sources

Tip 2

Wipe it with alcohol, then let it dry all the way

The trick

After scraping, wipe the joint with rubbing alcohol to take off the last soap and grease, then let it dry completely before the new bead goes on.

Why. Soap film and body oil keep sealant from grabbing the surface, and so does water left in the joint.

The tell. Old bead came off clean, tile bare under it? It never grabbed: soap film or a wet joint. Caulking a shower the same hour somebody used it is how that happens. Family Handyman speeds the drying with a hair dryer; GE says let it dry completely.

GE allows a mild cleaner or rubbing alcohol; a plumbing trainer Real Homes quotes uses rubbing alcohol and a paper towel for the last greasy residue; Family Handyman does a final wipe with mineral spirits. Mold in the grout along the joint? Family Handyman scrubs it with mold killer, rinses and lets it dry before any caulk goes on.

When to stop. Mold comes right back in the grout after you killed it, or the grout crumbles along the tub? Water is getting behind the tile. Caulk over that seals the wet in. The grout or tile gets fixed first.

Sources

Tip 3

Deep gaps get backer rod, and caulk sticks to two sides only

The number

Backer rod 1/8 inch bigger than the joint width. Sealant depth half its width (2 to 1), no less than 1/4 inch deep and no more than 1/2 inch (Sika).

Backer rod and the 2 to 1 caulk joint Cut away of a deep caulk joint. Closed cell backer rod 1/8 inch bigger than the gap sits below the sealant. The sealant is half as deep as it is wide, at least 1/4 inch and at most 1/2 inch deep, and bonds to the two sides only, not the bottom. CAULK JOINT, CUT AWAY width depth Sealant Backer rod The rod is the bond breaker. Rod: 1/8 in bigger than the gap. Depth: half the width (2 to 1). At least 1/4 in, at most 1/2 in deep. Two sides stick. Rod frees the bottom.

Why. Stuck to the two faces only, the bead can stretch as the joint moves. Stuck to the bottom too, it can't. The rod is the bond breaker, and it sets the depth.

The tell. A deep gap packed solid with caulk is the mistake. Press the rod in with something blunt; Weber says don't puncture, over-compress or stretch it.

Both sheets are for closed cell rod, and both say its job is to stop three-sided adhesion. Weber's own size table pairs rod to joint width in millimeters, so if your rod came with a table, follow it.

When to stop. Gap at a tub or shower wider than your thumb, or growing? Find what's moving, the tub or the wall, before you seal it. Caulk won't hold it together.

Sources

Tip 4

Fill a plastic tub before you caulk it

The number

Fill an acrylic or fiberglass tub about 70 percent with water, caulk it, let the caulk dry, then drain it.

Why. The water's weight sags the tub the way a person in it does. Caulked loaded, the bead gets compressed when the tub empties, The Money Pit says, so it won't fall out.

The tell. Bead cracked along the tub edge early on? It was probably caulked empty.

Not everybody does it. The 70 percent figure is from a plumbing trainer Real Homes quotes; an interior designer in the same article says there's no need. A sturdy cast iron tub doesn't need pre-filling at all, per Real Homes.

When to stop. Tub that moves when you step in, or flexes at the floor? That's support under the tub, and caulk will crack however you fill it.

Sources

Tip 5

Silicone where it's wet, latex where it's painted

The trick

100 percent silicone for showers, tubs and sinks; it is not paintable. Acrylic latex where you will paint. Siliconized acrylic in between: still paintable, with more flex and water resistance.

Why. Silicone is the flexible, mold-resistant one for places that stay wet, and This Old House says plainly it isn't paintable. Latex is the one paint sticks to; that's what it's for.

The tell. Paint peeling off a bead of trim caulk? Somebody used silicone. Read the tube: "siliconized" is acrylic with silicone added and still takes paint; "100 percent silicone" never will.

In a bathroom, Family Handyman calls for caulk with mold and mildew prevention additives. Swapping one kind for the other? Every bit of the old bead comes out first; GE says fresh sealant won't bond properly to existing material.

When to stop. Black coming back under fresh silicone in a shower? Water is behind the bead, in the grout or the wall. No tube fixes that; find where it's getting in first.

Sources

Tip 6

Tape the gap, cut the tip to match, wet finger from the corner

The trick

Two rows of tape spaced by the gap. Cut the nozzle to the gap width. Fill only the gap, tool it with a wet finger from an outer corner, and pull the tape while the caulk is still wet.

Why. The tape sets the edges, the tip sets how much goes in, and the finger rounds the bead. Let it skin over and the tape drags the edge with it.

The tell. A bead smeared wide past the tape lines is too much caulk out of too big a hole. Family Handyman likes a blunt 20 degree cut over 45.

Shower has to be used tonight? Don't caulk it tonight. A plumber Real Homes quotes says 30 minutes before water and 24 hours before you touch the bead, and the tube label wins.

When to stop. Gap too deep for one bead to fill? Don't keep stacking caulk in it. A deep gap gets backer rod first (see above).

Sources

The Old Tradesman is a cartoon character. Every tip on this page stands on the sources listed under it. He is the voice. They are the authority. The drawings are diagrams, not photographs.

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