So you built your own solid body guitar out of raw wood and you want to paint it so it looks like it has a professional finish on it. Because guitars usually receive a moderate amount of abuse, you must seal the guitar with a polyester clear coat after you paint it. This is the only way you can get a professional finish.
Prep the wood surface with pore filler, let the filler cure and then sand the front, back and sides flat with 600 grit sanding block.
Spray the guitar with primer in even thin coats until the entire guitar is covered with a flat layer of primer. Give the primer an hour to dry and then inspect the entire guitar for any scratches, dents, divots or other imperfections. If you locate any, fill the imperfections with pore filler, wait for the filler to cure and then repeat the priming process. Continue this until all of the imperfections are gone.
Sand the primer with a 600 grit sanding block. This gives the paint a surface to adhere to and prevents the paint from peeling up later.
Spray the guitar with paint in even thin coats until the entire guitar is covered with a flat layer of paint. The drying time depends on the humidity, brand and type of paint you use. Follow the directions on the paint can for drying times. Apply a second coat of paint in the given time frame on the paint can. Never use house paint. Use enamel paint or automotive paint.
Wet sand the orange peel off the paint surface with a 1000 grit sanding block. Orange peel is the textured paint left on the surface of the guitar that feels rubbery or tacky. Sand the surface until the surface is flat.
Spray the guitar with a coat of polyester or nitrocellulose hard coat. Again, the drying time depends on the humidity and brand of hard coat you use. Follow the directions on the can. Apply at least four thin coats of finish.
Wet sand the orange peel off of the hard coat with a 1000 grit sanding block until the finish is level.
Buff out the micro scratches with a buffing wheel and polishing compound.