Paint prep guide

Sandpaper for paint prep

Primer sanding and the sequences that keep paint from peeling.

Paint prep is where most painters lose the plot. Too coarse and you're leaving scratches the topcoat has to hide. Too fine and you're spinning wheels on a surface the primer won't grip. The grit you choose decides whether the paint that follows sits flat for years or starts peeling in months.

Paint prep vs. paint removal: not the same ladder

Paint prep is smoothing and de-glossing to help primer bond. Paint removal is stripping old coats off entirely. They live in different grit territories and demand different approaches.

Paint prep is usually 120 to 220 grit. The goal is to break the surface sheen and sand out imperfections (drips, nibs, dust) without cutting into the substrate underneath.

Paint removal starts coarse: 40 to 80 grit. You're attacking old paint, lifting it, and clearing the way to bare wood or primer. Once the old coat is gone, you step up to prep grits (120–220) to smooth what remains.

This guide covers prep. If you need to strip, start with the coarse grits and work your way up.

Drywall (primer prep and primer between coats)

Primer on drywall is what glues everything together. The surface underneath has to be smooth enough that the primer can flow without highlighting every imperfection.

100-120 grit: If the drywall is freshly mudded, start here after the mud hardens. A drywall pole sander with a 120-grit sheet or screen is the fastest way forward. Light pressure. Let the mesh do the work. On ceilings, it's not optional.

150-180 grit: Before primer goes on, one final pass with 150 or 180 kills any scratches from the 120 step. A sanding sponge works fine on walls and ceilings. You're looking for a uniformly matte surface under angled light. Once you see that, primer is ready to bond.

Drywall screens are your friend here. They're more aggressive than they look and load slower than sandpaper, so you get more mileage between passes.

Wood trim (primed or bare)

Wood is forgiving compared to other surfaces, but it's also where people wreck the finish by overthinking it.

Bare wood before primer: 120 grit raises the wood grain slightly, which helps primer key into the surface. Sand with the grain. One pass is enough. Wipe off the dust afterward. Primer goes on top.

Primer before topcoat: 150 to 220 grit, depending on how smooth you need it. Use a sanding sponge on any contoured trim. Flat surfaces take a sheet with a sanding block. The goal is to scuff the primer surface so the topcoat won't flash or sit proud.

Do not sand through the primer. If you do, seal the spot with primer again before topcoat.

Metal: bare and primed

Metal prep is finicky. Rust and mill scale are the enemies. Primer won't hold on top of either.

Bare metal (after rust removal): If you've already ground or blasted off rust, use 150 grit to smooth the surface left behind. The goal is something uniform, not shiny. Bare metal oxidizes fast, so prime the same day.

Primer on metal: 220 grit, a sanding sponge, and light pressure. You're not removing the primer, just roughing it so the topcoat grips. Scuff it evenly. Topcoat bonds to matte, not gloss.

Plastic and composites

Plastic doesn't accept primer like wood and metal do. You need to scuff it so primer has something to grab.

Before primer: 150 to 180 grit. A sanding sponge is ideal because plastic is softer than you expect and sponges won't gouge it the way sheet sandpaper can. Work gently. The point is to break the surface gloss, not to reshape the part.

After primer on plastic: 220 grit, same approach. Light passes. Uniform scuff. Topcoat next.

Between-coat sanding: when and how

The question everyone asks: do I have to sand between coats. The answer is not always, but when the surface isn't perfect, sanding between coats is how you fix it.

When sanding between coats is mandatory

Runs, sags, or drips: Wet sand them out with 150 to 180 grit and a sponge once the coat hardens. Let the paper follow the surface contour. Once it's level, the next coat will hide the repairs.

Dust or debris in the coat: 220 grit, light pass, remove the speck and the texture around it. Then topcoat again.

Gloss or semi-gloss primer: If your primer laid gloss, it's too slippery for topcoat to bond. Sand it with 150 or 180 to break the shine. Sanding sponge, uniform coverage, matte finish. Then topcoat.

When sanding between coats is optional

If the surface is smooth, matte, and clean, many topcoats bond fine without sanding. Check the product data sheet. Some primers and paints are engineered to bond to the coat beneath without it.

But if you're not sure, sand it. Ten minutes of sanding between coats saves six months of looking at a finish that's starting to peel.

Tools for between-coat sanding

Sanding sponges (120–220 grit): The workhorse. They follow contours, flex into corners, and don't gouge soft primers. Wet or dry, depending on the product.

Sanding sheets with a block (120–220 grit): Faster on large flat surfaces. Use a firm sanding block to keep the sheet flat. Don't let your hand conform; the block is the reference.

Orbital sanders (120–180 grit): A 5-inch random orbital sander speeds up large panels, but it's easy to over-sand with one. Start at low speed (around 3,000 rpm), check your progress frequently, and move on once the surface is uniformly matte.

Clear coat sanding (wet sanding the final finish)

Clear coat sanding is a different world from primer prep. It's slower, wetter, and less forgiving.

400 grit: The first step after clear hardens (wait 48 hours minimum, check the product sheet). 400 knocks down orange peel, dust, and runs. Keep it wet. Work slowly. Check your progress every few passes under angled light.

1000 grit and beyond: If you're polishing to a show finish, 1000 grit wet-sanded and compound-polished gets you there. See the wet-sanding guide for the full sequence. Paint prep doesn't usually demand this level. Standard topcoat doesn't need it either.

Common mistakes

• Sanding too coarse on primer. 80 grit on primer leaves scratches topcoat cannot hide. 120 is the floor for primer prep, and even that's aggressive.
• Not sanding off the primer gloss. Gloss primer doesn't accept topcoat well. If your primer is shiny, scuff it matte before topcoat goes on.
• Orbital sander on too high a speed. 3,000 to 5,000 rpm is the limit on primers. Higher speeds burn through and create swirls.
• Skipping primer to sand directly on bare surface. Primer is the interface between substrate and topcoat. Sand the substrate to prep it for primer, then sand the primer to prep it for topcoat. Don't skip either.
• Using the same grit the whole time. Primer prep usually needs a two-step sequence: medium (120) to flatten, fine (180–220) to finish. Skipping the fine pass leaves scratches.
• Not letting primer or topcoat cure before sanding. Wet primer or paint gums up the sandpaper and won't sand level. Wait the full cure time.

The grit sequence, recap

See the full grit chart for the complete guide:The complete sandpaper grit chart.