This story covers the procedures and tools available to prepare wood for painting.
Make the wood as smooth and perfect as possible to ensure a great finish. Find and fill defects, conceal unsightly edges in plywood, and sand thoroughly.
Natural defects in wood include tiny, solid knots; thin splits or cracks; and minuscule pest holes. Inspect your wood carefully for these and note them. If you find them on hardwood that will be stained and clear finished, wait until after you've completed the finishing to take care of them. Then apply a putty that matches the final finish. If you repair blemishes before staining and finishing, the repair will finish differently than the wood and will be more obvious.
One way to fill small gaps and minor imperfections before applying the clear finish is to mix some of the wood's own sawdust with a bit of the finish and fill with that. Fine dust from sanding is best; the dust-collecting bag or cup of a finishing sander is a great source. You can also purchase a commercially prepared stainable filler in a matching color.
Use a commercial filler or exterior patching compound to smooth the rough edges of softwood plywood you plan to paint. Then sand the repair. For hardwood plywood, use iron-on veneer tape of the same wood species. In some cases moldings can both conceal the edge and add style.
Do all final sanding with orange-color, open-coat garnet sandpaper. Dust won't clog it as easily as closed-coat papers, and the garnet particles fracture as you use them, producing an increasingly finer grit. For hand-sanding, "A" weight paper works best. Wrap it around a sanding block so the surface you're working on remains flat as you smooth it.
The higher the grit number, the finer the grit. For most work, start with 100-grit, then use 150-grit, and end with 220-grit. Clean the surface of the wood between sandings with a vacuum, a tack cloth, or a paper towel lightly dampened with a solvent such as mineral spirits.