“You can fix that yourself,” my friend said, pointing at the black-edged caulk around my tub. “It takes like an hour.” I laughed. Me? Caulk? I had never held a caulk gun in my life. Then she handed me a four dollar tube and a putty knife, and an hour later I had a bathtub that looked new. Here is the whole process.

Get the old caulk out

The black edges are mold growing in the old sealant, and scrubbing will not fix it. You have to remove the caulk completely:

1. Cut through the corners with a utility knife to break the seal.
2. Pull the old caulk out with a putty knife or a caulk removal tool. Long strips come out clean if you grab an end and pull slowly.
3. Scrub the area with a bleach cleaner and let it dry completely. If any mold is left on the tile, the new caulk will grow it right back.

Dry is the secret

Caulk will not stick to wet surfaces. I learned this the hard way, my first attempt peeled off within a week because I applied it over a damp edge. Wipe the area with a dry cloth, then let the bathroom air out for fifteen minutes. If you want to be extra sure, hit the gap with a hair dryer on low for a minute.

Apply it like a pro

Cut the nozzle of the caulk tube at a 45 degree angle, small opening first. Run a line of caulk along the gap, then smooth it with a wet finger or a caulk finishing tool. Press firmly so the caulk fills the gap completely, not just sits on top. Wipe the excess off with a damp cloth as you go.

Two details make the difference: masking tape on both sides of the gap gives you a straight line, and you pull the tape off while the caulk is still wet. I skipped the tape on my first attempt and got wavy edges that I had to trim with a razor.

Let it cure

Do not use the shower for 24 hours. The tube says twelve, but I have learned to double it. Caulk that cures fully stays flexible and watertight for years. Caulk that gets wet too early turns white and crumbly within months.

My friend was right, the whole job took about an hour, most of it waiting for things to dry. The black edges are gone, the tub looks clean, and I have not thought about it since.