Tech Pages - Do it Yourself Paint - Customs by Ripper
Since I see this question asked quite a bit, I'll describe quickly how to "do it yourself".
Before doing any prep work, clean the part with Dawn dishwashing soap and water. Then, when dry, hit it with wax and grease remover. This will remove all of the old dirt, grime and built up wax which could be forced into the paint by sanding. If this is a brand new motorcycle part from Drag Specialties or similar discount parts source, make sure you get all of the shipping oil off or your paint will not stick.
After the wax and grease remover, wipe the part down with a dry paper towel or clean rag (I cannot stress enough that this must be done with an absolutely clean rag. I use Bounty paper towels because they have less lint than the others. I used to buy expensive disposable shop rags, but they were just paper towels anyway.) You can use old, clean t-shirts as well.
Once clean and dry, you can begin sanding. If this is bare metal, you can sand with 40 on a D.A. (Dual action sander) If you are sanding by hand, use 80 grit. If you are just scuffing down existing paint to re-shoot, you can sand with 400 or 600 wet, D.A., or dry, by hand. If you choose to wet sand, use a plastic squeeze bottle filled with water and a few drops of dishwashing liquid to wet the surface. Keep the paper as wet as possible and check your progress by pulling a rubber squeegee across the surface. You'll be able to see where you need more sanding as well as where there are problems like scratches, small dings, etc. with this method.
Primer. This is where many arguments happen, but... I've been painting cars, trucks, motorcycles, etc. for 40 years and I've seen the worst that can happen, so take this with the many years of experience that it comes with. You do not need primer unless you are working on bare metal, you have made repairs or you need a primer/sealer to hold down something that might be an issue. I personally do not like using primer/sealer to avoid sanding bad paint off, but there are some who choose to do this. I warranty all of my work, so this is not an option as far as I'm concerned. All parts get stripped to metal unless I can validate that the surface will not cause a future problem (like original factory paint that we are just airbrushing over or fresh paint applied by a trusted source, etc...., everything else gets stripped). There is no reason to shoot primer over paint, ever. The only exception is for a repair, where you would need to feather out the repair area into the existing paint work.
Sand your primer with 600-800 wet, dry or on a D.A., being careful not to cut through to the metal or undo your repair. If you do cut through, spray at least one more coat of primer, over the damaged area, then sand that area again. A final light scuff with 800 grit and you're ready to paint. Repairs are not covered in this discussion, but if anyone would like, we could go there in another article.
Paint. Follow the directions on your paint for mixing, spraying, tack times, etc. If you are doing a spray can special, you won't need to worry about mixing paint. Apply color until all of your repaired areas or all of the primer is fully covered, then go one more coat. When this is dry, you may wet sand with 800-1000 and give one or two more coats. Do this only if necessary. If you don't have any problems to address, you can shoot your clear as soon as the base is dry enough. This varies by paint manufacturer, but it's usually between 5 minutes to an hour that you have as a window for best adhesion (some basecoats can be cleared up to 24 hours later without scuffing). Check with your paint supplier for any additional tips regarding mixing, spraying, clearing, etc. They all have data sheets they will give you for free, just ask your supplier. It is best to use an autobody supply jobber for this, as they deal with this on a daily basis. One last tip in this section. If you need to sand your basecoat, make sure you spray at least one more coat over your sanding marks prior to hitting the part with clear. If you do not, your sanding marks will show through the clear, especially with a metallic or pearl base. That would not be good. Not good at all.
Clear. This part of the process differs by brand as well as product type, so the best I can do here is say check with your supplier for a data sheet on your choice of clear (or ask here, you never know. I've used just about every paint system out there). Once your clear is cured (overnight for most urethane clears), you can polish it out. That is also a topic for another discussion.