Customer has a bathroom ceiling with mildew spots.

Stop the moisture first. Mildew returns if the source isn't fixed. Verify the bath fan vents outside (not the attic), runs 20+ min post-shower (IRC M1507.3), and moves ≥50 CFM intermittent/20 CFM continuous. Check duct for kinks/blockage.

Kill & remove existing growth:

1. PPE: N95/respirator, goggles, gloves. Ventilate.
2. Clean: Scrub with 1:3 bleach:water (or commercial mildewcide like Concrobium/Jomax) using stiff brush. Never mix bleach with ammonia/acids.
3. Rinse: Thoroughly with clean water. Let dry 24+ hrs (verify <15% moisture with meter).

Prime: Mandatory. Use shellac-based (BIN) or high-quality oil/alkyd stain-blocking primer (Zinsser Cover Stain, Kilz Original). These seal residual stains/spores best. Water-based "mildew-resistant" primers alone won't block bleed-through. One full coat, back-roll.

Paint: 100% Acrylic satin/semi-gloss with added mildewcide (or factory-fortified bath paint: Benjamin Moore Aura Bath & Spa, Sherwin-Williams Duration Home). Two coats.

Pro Tip: Caulk perimeter (tub/shutoff) with 100% silicone after paint cures. Ensures vapor barrier continuity.

If drywall is soft/crumbly: Cut out, replace, tape, mud, prime, paint. Painting over compromised rock fails.

To prevent mildew from returning through new paint, you must eliminate the existing growth and address the underlying moisture issue. Here’s a step-by-step, code-informed approach:

1. Identify and Fix Moisture Source Mildew in bathrooms is caused by excess humidity. Ensure proper ventilation: - Verify the exhaust fan is sized correctly (minimum 50 CFM for bathrooms under 100 sq ft, per IRC M1507.3) and vents to the exterior (not attic or soffit). - Run the fan during and 20 minutes after showers. Consider a humidistat switch for automatic operation. - Check for leaks in plumbing, roof, or around fixtures.

1. Clean the Affected Area - Wear gloves, goggles, and an N95 mask. - Scrub the ceiling with a solution of 1 part bleach to 3 parts water (or use an EPA-registered mildewcide like Concrobium). - Let sit 10–15 minutes, then rinse thoroughly with clean water and allow to dry completely (24+ hours). - Do not just paint over mildew—it will grow through the paint.

1. Prime with a Mold-Resistant Primer - Apply a stain-blocking, mold-inhibiting primer such as Zinsser Mold Killing Primer or Kilz Mold & Mildew. - These primers contain EPA-registered antimicrobials and seal residual spores. - Follow manufacturer’s drying time (usually 1 hour).

1. Paint with Mildew-Resistant Paint - Use a high-quality bathroom paint labeled “mildew-resistant” (e.g., Benjamin Moore Aura Bath & Spa, Sherwin-Williams Duration Home). - These contain mildewcides and are formulated for high-humidity areas. - Apply two coats for durability.

Important: If mildew is widespread, recurring, or you suspect hidden mold behind drywall, consult a licensed mold remediation specialist (per EPA guidelines). Never ignore persistent moisture—it can lead to structural damage or health issues.

Bottom line: Kill the mildew, fix the ventilation, prime, then paint with the right products. Skip any step, and it’ll come back.