Caulk a tub and a window so nobody can tell you did it

Quick answer: Get every scrap of the old caulk out, let the joint dry for a full day, tape both edges, and fill the tub with water before you run the bead. Pull the tape while the caulk's still wet.

You've seen the bad version. A gray, lumpy line around the tub that looks like somebody piped frosting with their eyes closed. It's peeling at one corner, and there's a dark shadow creeping underneath it. Somebody ran a fresh bead right over the old one and called it done on a Saturday afternoon.

Nobody's hands are that shaky. That job failed before the gun came out.

A tube of decent silicone costs less than lunch. The materials are never the problem. The difference between a joint that lasts years and one that peels by Thanksgiving is about two hours of prep that most people skip because it's boring.

The bead is the easy part

Most people think caulking is a hand-skill job, like icing a cake. It isn't. Caulk only does two things. It sticks to clean, dry surfaces, and it stretches a little when those surfaces move. If either one is missing, no technique on earth will save it.

So the order of effort runs backward from what you'd guess. Removal and cleaning take the longest. Taping takes ten minutes. The bead itself takes about ninety seconds a side.

Get every last bit of the old stuff out

New silicone won't stick to old silicone. It won't stick to the invisible film old silicone leaves behind, either. That's why caulk-over-caulk jobs peel. The new layer is sitting on a surface it can't grab.

Score both edges of the old bead with a utility knife, then pull. On a fiberglass or acrylic tub, use plastic razor blades. A steel blade will scratch that finish, and a scratch is permanent. A 5-in-1 painter's tool works well for prying out the stubborn bits in the corners.

If it won't budge, a caulk remover gel like DAP Caulk-Be-Gone softens it up. Give it the time on the label. Don't rush it.

Then wipe the whole joint with isopropyl alcohol on a clean rag. Do it twice. This is the step everybody skips, and it's the one that matters most.

If you find black mold behind the old caulk, clean it out completely before you go any further. Sealing over it just builds the mold a nicer house.

Now the annoying part. The joint has to be bone dry, and that means a day. Run a fan, crack a window, and use the other shower. Moisture trapped behind new caulk is how you end up doing this twice.

Fill the tub before you caulk

This is the trick that separates the guys who've done this before from everybody else.

An empty tub sits a little high. Fill it with water and a person, and it settles. If you caulk it empty, the first real bath pulls the tub down and tears the fresh joint open along the wall. You won't see it at first. The water will find it.

Fill the tub to the overflow before you start, and leave it full until the caulk has cured. Check the tube for the water-ready time. Then drain it. Now the joint is sitting at its loaded position, and every time the tub empties, the caulk is under slight compression instead of tension. Caulk handles that much better.

Tape, cut, run, tool, pull

1. Tape both sides. Run painter's tape along the wall and along the tub, leaving a gap a little wider than the joint itself, roughly an eighth of an inch back on each side. Press the edges down hard with a fingernail. Straight tape gives you a straight line. That's the whole secret to the "professional" look.
2. Cut the nozzle small. Cut it at an angle, smaller than you think. You can always cut more off. You can't put plastic back. Punch the inner seal with the wire on the gun.
3. Run one continuous bead. Steady pressure and steady speed, one wall at a time, corner to corner. Don't stop in the middle if you can help it. Stops leave bumps.
4. Tool it once. Dip a finger in soapy water, or use a cheap plastic caulk tool, and draw it along the bead in one smooth pass. You're pressing the caulk into the joint and shaping it. You're not trying to wipe most of it away.
5. Pull the tape now. Not in an hour. Now, while it's wet. Pull at a low angle, away from the joint. The edge it leaves will be crisp.
6. Walk away. Don't touch it, don't "fix" a spot, don't test it with a fingertip. Every touch after tooling makes it worse.

A dripless caulk gun is worth the few extra dollars. Cheap ratchet guns keep pushing caulk after you let go, and that's where the blobs at the end of every run come from. It's on the short list in Ten tools that cover ninety percent of household repairs for good reason.

A window is two different jobs

Inside and outside use different caulk. Mixing them up is the most common window mistake.

Inside, on the trim. You're closing gaps between the casing and the wall so the paint looks clean. Use a paintable acrylic latex, like DAP Alex Plus. It cleans up with water and takes paint. Silicone won't take paint. Put silicone on interior trim and you'll find that out the hard way. If you're painting the room afterward, The paint job that still looks right in three years covers what comes next.

Outside, around the frame. Now you're keeping weather out, and the joint moves with every hot afternoon and cold night. Use something rated for exterior use that stays flexible. OSI Quad Max and Sashco Big Stretch are both good choices. Check the tube's minimum application temperature, and don't caulk if rain is coming before it skins over.

Three exterior rules people break all the time:

• Use backer rod on wide gaps. If the gap is wider than about a quarter inch, push in foam backer rod first. A good joint sticks to two sides, not the back wall too. Caulk stuck on three sides can't stretch, so it tears.
• Don't seal the weep holes. Many windows have small slots along the bottom of the frame. They're there to let water out. Caulk them shut and you've built a little dam inside your window.
• Don't caulk the sash. You're sealing the frame to the house, not the moving parts. A window that's caulked shut is a window you can't get out of in a fire.

What we'd buy, what we'd skip

Buy:

• GE Silicone 2 Kitchen & Bath for the tub. It's 100 percent silicone and made for wet areas.
• DAP Alex Plus for interior trim you plan to paint.
• OSI Quad Max or Sashco Big Stretch for exterior windows.
• A dripless caulk gun and a roll of good painter's tape.

Skip:

• Acrylic "tub and tile" caulk in a shower that gets daily use. It's easier to work with, and that's a real advantage for a first-timer. It just won't last like silicone in standing water.
• Any plan that involves caulking over old caulk.
• Clear silicone if you're nervous. Clear shows every smear and every bubble. White hides your learning curve.

When it isn't a caulk problem

Caulk seals a joint. It doesn't fix the thing behind the joint. Stop and get someone qualified to look if you find any of these:

• The floor near the tub feels soft or flexes when you step on it.
• Tiles are loose or sound hollow when you tap them.
• The gap keeps getting wider every time you recaulk.
• The window sill is soft, dark, or crumbles under a screwdriver.
• The house was built before 1978 and you'd be scraping old paint. Lead-based paint was common before then. Get it tested before you sand or scrape.
• The exterior window is on the second story and you'd be working off an extension ladder you don't trust.

Those are water damage, structure, or safety problems wearing a caulk costume. Knowing when to stop and how to get a quote you can trust walks through hiring for exactly that moment.

One last thing. If you rent, redo the tub before you hand back the keys, even if it was already ugly when you moved in. It's an afternoon and a tube of silicone. Landlords remember the tenant who left the bathroom better than they got it, and that's a reference you'll want someday.

A good caulk line is invisible. Nobody walks into your bathroom and compliments it. That's how you know you got it right.