Short answer
To caulk around a bathtub or sink, remove all old caulk, clean and dry the joint, mask with painter’s tape, apply a steady bead of bathroom-rated silicone, tool it smooth, pull the tape, and let it cure fully before getting it wet. The keys are clean surfaces, the right product, and patience during curing.
Why this matters
A fresh, properly applied bead of caulk keeps water out of seams where tubs, showers, and sinks meet walls or countertops. That prevents mold, soft drywall, swollen cabinets, and costly repairs. It’s a satisfying 1–2 hour DIY that most homeowners can handle with a few common tools.
Tools and materials
- 100% silicone "Kitchen & Bath" caulk (mildew-resistant). Choose clear, white, or color-matched. For natural stone, use neutral-cure silicone.
- Caulk gun (smooth-rod, dripless style recommended)
- Painter’s tape (1/2–1 inch)
- Utility knife with fresh blades and/or a plastic caulk removal tool
- Caulk remover gel (optional but helpful on stubborn silicone)
- Rubbing alcohol (70–90%) and lint-free rags
- Paper towels and a small trash bag
- Nitrile gloves and safety glasses
- Closed-cell foam backer rod (for gaps wider than 1/4 inch)
- Caulk-smoothing tool or a gloved finger
- Small vacuum or brush
Estimated cost: $25–$50 if you’re buying everything; $6–$12 for a tube of quality silicone.
Choose the right caulk
- Tubs/showers: Use 100% silicone labeled "Kitchen & Bath" and "mold-resistant." It stays flexible and resists constant moisture.
- Sinks/countertops: Silicone works great. If you need to paint nearby trim later, use a paintable silicone-hybrid only on non-wet surfaces.
- Natural stone (marble, granite): Use neutral-cure silicone to avoid staining.
Coverage: A 10 oz tube typically covers 20–30 linear feet at a 3/16 inch bead.
Joint guidelines
- Gap width: up to 1/4" = caulk only
- 1/4" to 1/2" = add backer rod sized 25–30% larger than the joint
- Bead size: 1/8"–3/16" concave profile
- Cure time: 24 hours typical for 100% silicone (check label; some are water-ready in 30–60 min)
Step-by-step: Bathtub or sink
1) Prep and remove old caulk
- Ventilate the room and wear gloves/safety glasses.
- Cut along both edges of the old bead with a sharp utility knife. Use a plastic scraper or caulk removal tool to lift it out. Apply caulk remover gel if needed; follow dwell time instructions.
- Scrub away residue. Do not leave any old silicone—it prevents adhesion.
- Kill any surface mildew with a bathroom mildew remover, rinse thoroughly, then let fully dry. Do not mix cleaners.
2) Clean and dry
- Wipe the joint and adjacent surfaces with rubbing alcohol to remove soap film and oils. Let dry completely. A fan helps.
3) Manage movement
- For acrylic/fiberglass tubs, you can fill the tub with water before caulking to load the joint slightly. Drain after the caulk has skinned. This reduces future cracking from movement.
- For wide gaps, press in backer rod flush or slightly recessed.
4) Mask for crisp lines
- Run painter’s tape along both sides of the joint, leaving a gap as wide as your desired bead (generally 1/8–3/16 inch). Press tape edges firmly.
5) Cut the nozzle and load the gun
- Cut the tip at a 45° angle to a small opening—start around 3/32–1/8 inch. Puncture the inner seal if present.
- Practice on scrap cardboard to dial in flow.
6) Apply the bead
- Hold the gun at about 45°. Keep steady pressure and a constant speed.
- Aim to push the bead into the joint (move the gun so the nozzle slightly compresses the caulk), which helps wet out both sides.
7) Tool the bead
- Within 5–10 minutes (before skinning), smooth the bead with a caulk tool or a gloved finger. Lightly misting your finger or tool with a 50/50 water–isopropyl alcohol mix can reduce sticking. Don’t flood the surface.
- Aim for a smooth, slightly concave profile that spans both sides of the joint.
8) Pull the tape and tidy up
- Remove painter’s tape immediately, pulling back on itself at a 45° angle while the caulk is still wet.
- Spot smooth edges with a clean, dampened fingertip if needed. Wipe stray smears with alcohol.
9) Cure
- Respect the product’s cure time. Avoid water and cleaning for the full cure—often 24 hours, even if the label says "water-ready" sooner.
Safety and handling
- Work in a well-ventilated area; acetoxy silicones have a vinegar-like odor.
- Cut away from your body; change blades frequently to prevent slips.
- Do not use harsh solvents on acrylic tubs or plastic trim.
- Never mix cleaning chemicals (bleach + ammonia or acids).
Tips for best results
- Warm tube = smoother flow. Store at room temperature.
- Keep a dry rag on one side of your non-dominant hand and an alcohol-dampened rag on the other for quick cleanup.
- Caulk after the last coat of paint and final cleaning to reduce contamination.
- Match colors: white hides soap scum, clear disappears on glass/stone, color-matched silicone blends with grout and tile.
Common mistakes to avoid
- Using latex painter’s caulk in wet areas—it won’t last.
- Caulking over damp, dirty, or soapy surfaces.
- Skipping removal of old silicone; new silicone won’t bond well to old.
- Cutting too large an opening and laying an oversized bead.
- Not using backer rod on wide joints, which causes deep, three-sided adhesion and premature failure.
- Over-tooling with too much soapy water, which can weaken adhesion.
Special notes for sinks
- Drop-in sinks: Run a thin bead under the rim before setting, then a finish bead around the perimeter after tightening. Don’t block the overflow.
- Undermount sinks: Follow the manufacturer’s silicone spec; support and clamp as directed and allow full cure before loading.
When to call a pro
- Repeated caulk failure or large recurring cracks (movement or substrate issues).
- Soft, spongy walls or floors near the tub (possible hidden water damage).
- Gaps wider than 1/2 inch, missing tiles, or loose tub—these indicate repairs beyond caulking.
- Natural stone installations where staining risk is high and a specific sealant is required.
With the right bathroom-rated silicone, careful prep, and a steady hand, you’ll get a clean, durable seal that keeps water where it belongs and looks sharp for years.