How to Caulk a Bathtub or Shower the Right Way (No Mildew, No Peeling)

Caulk is the cheapest, most-overlooked piece of waterproofing in your home — and when it fails, it fails expensively. Mildew creeps in, water seeps under tile, subfloors rot, and what should have been a $10 fix becomes a $5,000 demolition. Worse, most amateur caulking jobs look terrible and fail within a year.

This guide teaches the professional method: which caulk to buy, how to remove old caulk completely, and the single technique that produces a perfect seam every time. Master it once and you'll never have a moldy bathroom seam again.

Why Caulk Fails

• Wrong caulk type used (acrylic latex in a wet area is the #1 sin)
• Old caulk not fully removed before re-caulking
• Surface not dried completely before application
• Soap scum or oil residue left on tile
• Joint moved while caulk was curing
• Caulk applied too thin or in too wide a gap

Choose the Right Caulk

For tubs, showers, sinks, and any wet area, use 100% silicone caulk labeled for kitchen and bath. GE Silicone II Plus or Loctite Polyseamseal are reliable choices. NEVER use acrylic latex 'painter's caulk' in wet areas — it absorbs water, supports mildew, and fails in months.

Color

White is standard for tubs. Clear works on dark tile. Avoid colored silicones — they discolor over time. For grout-matching seams, use sanded acrylic-latex caulk only on dry vertical seams (never the tub line).

Tools You'll Need

• 100% silicone caulk for kitchen/bath ($6/tube, you'll need 1–2)
• Caulk gun (smooth-rod or dripless)
• Razor blade or oscillating multi-tool with caulk-removal blade
• Goo Gone or 3M caulk remover
• Painter's tape (1-inch)
• Paper towels and rubbing alcohol
• Caulk smoothing tool or just your finger
• Rags

Step-by-Step: Removing Old Caulk

This is 80% of the job. Skip it and your new caulk will fail within months.

1. Score the old caulk with a razor blade along both edges (against the tub and against the tile)
2. Lift one end with the razor and pull off in long ribbons
3. Use a plastic putty knife to scrape stubborn residue (metal scratches porcelain)
4. Apply Goo Gone or caulk remover gel to break down silicone residue (sit 15–30 minutes)
5. Wipe away with rags, repeat as needed
6. Wipe the entire area with rubbing alcohol to remove all oil, soap, and caulk residue
7. Let dry completely — at least 4 hours, preferably overnight

Crucial: ANY old silicone residue prevents new silicone from bonding. The new bead will look fine for a week, then peel right off. Take time on this step.

Step-by-Step: Applying New Caulk

Step 1 — Tape

Apply painter's tape to both sides of the joint, leaving a gap exactly the width you want the finished caulk bead — typically 3/16" to 1/4". This produces a perfectly straight line every time.

Step 2 — Fill the Tub (For Tubs Only)

Fill the tub with water to weight it down. The tub will deflect slightly when full of water and a person — you want it caulked in its 'loaded' position, not its empty position. Otherwise the caulk will tear when you next use the tub.

Step 3 — Cut the Tip

Cut the caulk tube tip at a 45° angle. Cut small first — you can always cut bigger. The hole should be slightly smaller than the gap you're filling.

Step 4 — Apply

Hold the gun at a 45° angle, pull (don't push) the gun along the seam at a steady pace, squeezing the trigger evenly. The bead should fully fill the gap with a slight crown above. Do one continuous run — stops and starts show.

Step 5 — Tool the Bead

Within 5 minutes of application, smooth the bead. Wet your finger with soapy water, run it along the bead in one continuous motion. The soap prevents the silicone from sticking. Wipe excess on a rag between strokes.

Step 6 — Remove Tape

Pull the tape immediately while caulk is wet — pulling at a 45° angle away from the bead. If you wait until caulk skins, the tape will tear the bead.

Step 7 — Cure

Don't get the bead wet for 24 hours. Drain the tub gently after 24 hours. Full cure takes 7 days — avoid heavy use until then.

Common Mistakes to Avoid

• Caulking over old caulk — never bonds
• Caulking when the bathroom is humid — caulk traps moisture
• Tooling the bead with your dry finger — drags caulk
• Filling a too-wide gap (more than 1/2") — use backer rod first
• Using acrylic latex in wet areas — guaranteed mildew
• Showering within 24 hours of caulking — water washes the curing caulk

What If There's Already Mildew?

If you can see black mildew under the caulk, it's IN the caulk and the gap behind it. Bleach surface treatments only mask the problem. Remove the caulk completely, clean with a 1:10 bleach-water solution, dry for 24 hours, then re-caulk. Use a mildew-resistant silicone.

Special Situations

Acrylic Tub Surrounds

Acrylic flexes more than tile. Use a flexible silicone (regular kitchen-and-bath silicone is fine, but avoid hybrid/sanded products).

Solid Surface (Corian)

Most are seamless and don't need caulk except where they meet the wall. Use color-matched silicone if available.

Stone or Marble Tile

Use a non-acidic, non-staining 100% silicone. Some silicones contain acid that etches marble — read the label.

DIY vs. Professional

Caulking is one of the highest-skill DIY jobs that looks easy until you try it. The first attempt is rarely perfect. If you're caulking a master bath you want to be perfect, or you're not confident, hire it out — a pro does it in 1 hour for $200 and it lasts a decade.

2026 Cost Estimates (Bay Area)

Maintenance

• Spray caulk lines with diluted hydrogen peroxide weekly
• Use a squeegee after every shower
• Run the bath fan during and after every shower (see our bath fan repair guide)
• Wipe up standing water in tub corners
• Re-caulk every 5–7 years even if it looks fine

When to Call a Pro

Persistent mildew that returns after re-caulking, signs of subfloor damage, missing or cracked tile alongside bad caulk, or any time you want it done right the first time. Our team services San Jose, Sunnyvale, Mountain View, and surrounding cities.

FAQs

Why does my caulk turn black?

Mildew. Either inside the caulk (must be removed) or on the surface (cleaned with bleach).

How long does silicone caulk last?

8–15 years with proper installation and ventilation.

Can I caulk over silicone?

No. Silicone bonds only to clean surfaces. Old silicone must be 100% removed.

Why does my caulk peel away from the tub?

Either applied while the tub was empty, or oil/soap residue prevented adhesion.

Is caulk waterproof?

100% silicone, when properly installed, is permanently waterproof.

Get a Bathroom That Stays Watertight

Call (510) 938-4277 for fast, professional re-caulking that lasts. Or browse our full home repair services.