Preparing a surface for painting involves making sure the surface is clean, free of debris, dirt and grease. Sanding a surface is part of the preparation, but it is also the most important part of the process if a smooth, shiny surface is wanted. It is said that preparation work for painting is 70 per cent sanding. The more a surface is sanded and the more time that is taken during this step, the smoother and better the paint will look when it's done.
Start Sanding Projects with 80-Grit Sandpaper
Most projects will need to be started with 80-grit sandpaper. This is a very coarse paper that will help to strip paint, remove large scratches and scuff up the surface, getting it ready for the next step. Depending on the delicacy of the surface, 80-grit might be too harsh. Plastics and other soft surfaces may need to have a finer grit used on them, such as 100 or 120. Try a few different pieces, using them gently at first to see what gives you the grit you desire for the job at hand. Sand the entire surface and bring it to as smooth a surface as possible.
Filling and Scratch Removal
If there are deep scratches, holes or dents in the surface that will be painted, those should be filled with appropriate materials. If the surface is drywall, use a drywall patch, it if is wood, use a wood filler, if it is metal, use bondo or other filler. Use these scratch fillers as directed on the package and let dry them completely. Use 80-grit sandpaper on the area that has been patched to smooth out the surface. Once it is fairly smooth, switch to a smaller-grit sandpaper to smooth it out again. Continue sanding with gradually finer paper, stepping down until the surface is very smooth to touch.
Wood and Fiberglass Finishes for Boats
Start with a rough-grit paper, such as 220 or even 100, if necessary, to get rid of any oxidation. Sand the entire surface, gradually increasing in grit to 320. The first coat or two can be put on and sanded with 320-grit. Once the surface has been smoothed with that, move to 400-grit and paint again. Final sanding on the very last coat should be 800-grit, and once a clear coat is applied, a once-over final sand to buff that paint job would be best done wet-sanded with 2,000-grit sandpaper.
Exterior Painting Wood & Trim
Exterior and Interior house paint to not require as smooth as surface as a boat or car, so use of sandpaper can be of a more coarse grade. Starting the process with 60- or 80-grit paper works well to remove old paint and scratches. Work up to 120-grit paper until the surfaces are smooth. Painting can be done at this point.
Fine Sanding for Wooden Crafts
Sanding for wooden crafts usually requires a very smooth and fairly shiny surface, but not as smooth as a boat or vehicle. Starting with 60- or 80-grit paper works for fine-tuning a wooden project, then move to 120-grit paper. Work your way to 400-grit for the final coat of paint.
Automotive Painting for a Deep Shine
When sanding a vehicle for painting, an extremely smooth surface is required. Sanding and increasing the grit grade until you reach 800-grit paper is recommended. Once you reach that and have primed the car, put on the first coat of paint and use 1,000-grit paper with a wet-sand technique between coats. After applying clear coat, continue with 1,000-grit wet sanding. A final clear coat can be finished with 1,000- or 2,000-grit wet sanding, depending on shine desired.
Painting on Motorcycles and Airbrushing
It is best with motorcycles to strip any paint off and start with bare metal. Priming and sanding an unpainted part starts with 600-grit sandpaper. Work your way down, priming and sanding in between coats until the finish is very smooth. After the first coat of paint, start using 800-grit paper and work your way to 1,000-grit wet sanding. Once a few coats of clear coat are on, switch to 2,000-grit wet sand and apply a few more coats, finishing with a buff with 2,000-grit wet sanding.