How to Prep Walls Before Painting: Clean, Patch, Sand, Prime and Caulk

What good prep achieves

Manufacturer training material reduces prep to four requirements: the surface must be clean, dull, dry and sound. Clean means no grease, dust or grime. Dull means glossy surfaces are sanded or treated with an adhesion primer. Dry means no moisture in the wall. Sound means nothing flaking, powdering or crumbling that paint could peel off with. A product data sheet adds a warning worth remembering: any prep short of total removal of old coating may compromise the service life of the system, so prep to a stable surface rather than just covering defects.

Tools and materials

You need a few simple categories of supplies.

• Mild soap, lukewarm water and a cellulose sponge.
• Lightweight spackle and a putty knife for holes and cracks.
• 120 grit sandpaper, plus a coarser grit for worn surfaces, and a tack cloth or damp cloth.
• Primer matched to the surface, including a stain-blocking primer if needed.
• Paintable acrylic caulk and a caulk gun.
• Painter's tape and drop cloths.

Step by step

• 1. Move furniture away, cover the floor and remove switch plates, outlet covers and register grills.
• 2. Wash walls and trim with a mixture of lukewarm water and mild soap, scrubbing in circles. Rinse with a slightly damp sponge.
• 3. Let everything dry completely before applying tape.
• 4. Scrape or sand loose or peeling paint to a sound surface. Patch holes and cracks, let the patch dry, sand smooth and wipe away dust. Joint compound must be cured and sanded smooth.
• 5. Sand glossy areas dull, or wash them and use an adhesion primer instead.
• 6. Spot prime patches, or prime the entire wall if it is bare, stained or changing color. Use a tinted primer under a darker color.
• 7. Caulk gaps between molding and wall with acrylic caulk after priming. Check the label first because some caulks are not paintable, then smooth the bead with a wet finger and wipe clean.
• 8. Tape trim with long strips and press the edges firmly with a putty knife.

Dealing with stains, mildew and moisture

Mildew looks like dirt but will not wash off, and it will grow through any new coat of paint. Scrub it off with a commercial mildew remover, following the product's safety instructions, before painting. For water-stained ceilings, correct the source of moisture first, then use a stain-blocking primer-sealer. The manufacturer says alkyd versions usually work better because they contain no water that could re-wet the stain. If a stain keeps returning, call a plumber or roofer before painting again.

Prepping different surfaces

Drywall: fill cracks and holes, sand and remove all sanding dust. Plaster: bare plaster must be cured and hard; soft or powdery plaster should be treated with a solution of 1 pint household vinegar to 1 gallon of clean water, repeated until hard, rinsed and dried. Wood: sand exposed wood to a fresh surface and patch with wood filler. Concrete and block: new surfaces are normally cured about 30 days; if you cannot wait, the data sheet allows 7 days with a masonry primer. Previously painted surfaces with oil-based paint can take latex if sanded and cleaned, or washed and primed with an adhesion primer.

Lead and older homes

If the home was built before 1978, do not dry scrape or power sand without precautions. See our lead-safe painting page and the EPA guidance before disturbing old paint.

Prepping trim

Trim needs its own prep. The manufacturer trim guide says to clean it with a damp sponge, sand and patch holes with lightweight spackle, and sand dry spackle smooth with 120 grit paper before removing dust with a tack cloth. Sand new or already smooth trim with 120 grit until all shine is gone. For rough or worn trim, start with 80 grit and then switch to 100 or 120 grit. Prime all bare wood and spackled spots and let the primer dry 24 hours, then lightly sand with 120 grit to remove brush marks and drips and wipe away dust. Apply a second coat of primer if bare wood or spackle still shows.

Choosing tools for prep and priming

For priming, the manufacturer prep page recommends a 2 inch or 2.5 inch angled brush along trim and inside corners, then a roller for the remainder of the wall. It lists a 3/8 to 1/2 inch nap for smooth surfaces and a 1/2 to 1 inch nap for rough or stippled ones. When taping, work in small sections from a corner, use longer pieces rather than several short ones, and firm the edge with a clean putty knife. Do not tape until walls and trim are thoroughly dry.

Common prep mistakes

• Taping to a wet or dirty surface so the tape does not stick.
• Painting before spackle is dry and sanded.
• Caulking before priming, or using caulk the label says is not paintable.
• Skipping stain-blocking primer on water stains.
• Leaving sanding dust on walls.

Frequently asked questions

Sources: Sherwin-Williams ProMar 200 Interior Latex Eg-Shel product data sheet (03/2025), Sherwin-Williams: How to Prep Walls for Painting, Sherwin-Williams: Interior Surface Preparation FAQs, Sherwin-Williams Painter Skills Program, Part 1 (training presentation), EPA: Information for Do-It-Yourselfers and the RRP Rule. Read 7 November 2026.