This is all assuming that you are using a Basecoat/Clearcoat (BC/CC) Urethane system over a previously painted surface that didnt' have any adhesion problems (the paint wasn't flaking off or hadn't seen 'Re-entry burn' like so many cars from the 80's). Some of this advice will apply for those using "spray can" painting, but the type of paint and technique used to spray from a can have many different properties than modern Urethane paint.
I'm assuming you're using modern spraying equipment:
-An air Compressor able to keep up with 8 to 20psi at .75 to 3 ft^3 per min. Except for spraying the polyester, most small "25 gallon, 5-horse" Craftsman aircompressors will work.
-Either an HVLP automotive-type spray gun, or an old-style Siphon-feed automotive-type spray gun with the correct sized tip (check with manufacturer's websites).
-A water trap and oil trap at the compressor, plus a disposable watertrap that screws on the base of the gun as last-chance-insurance against water getting in the paint
This is also assuming that you are shooting a solid color (or colors). I dont mean you can't put graphics or multiple solid colors on the peice, but that you aren't using "Transparent" or "Kandy" colors. The equipment, skill and knowledge required to shoot these with any success is far better left to someone who knows what they are doing. Putting a "run" in a solid color is easily fixed in a few seconds. Putting a "run" in a Kandy color usually requires starting completely over from Step 1.
First, to be able to do something correctly, you have to have a certain understanding of what you are doing. The Urethane paint is essentially a plastic in a liquid form. It is reduced (normally) 1 part paint to 1 part Urethane reducer (NOT laquer thinner). This type of paint adheres to whatever it is sprayed on in two ways: about 80% of the adhesion of a urethane paint is through a Physical bite. This means that the surface that you are spraying onto has to be ready Physically for the paint to adhere to it...usually it needs to be sanded by 600 grit or coarser (usually no more coarse than 320, or you'll see the sanding scratches after you are done). The other 20% of your bite is Chemical. That means there are chemicals in the paint that actually try to EAT into whatever your spraying onto and create a bond between the paint your spraying and the surface.
So...in preparation, you need a surface that is made to accept this "Chemical bite", but that is also sanded correctly to give the paint it's "Physical bite".
The best surface for this is either an Epoxy Primer or a Uretane Primer. I wont go into the differences, but we only use High-build Epoxy Primer. There are many makers of this type of Primer, varrying in price from a $10 or $11 per Qt (you wont need more than a quart for morst bikes) to over $50/Qt. It will require an activator, and will require VERY HIGH amounts of pressure to spray. Unless you thin it considerably with Urethane Reducer, this stuff is like Liquid Putty. In fact, we use a type of Epoxy Primer that is advertised as a Polyester Coat, and goes on EXTREMELY thick. It is put on as if we were covering the entire bike in a layer of putty. It sands VERY easily, and we can do bodywork on top of it, or just sand this material flat and skip putting regular putty on.
Here is where things can get complicated. IF you have bodywork to do, AND you are going to be using a normal "Putty", you will need to put a "Sealer" over the top of all of your bodywork after you are done. Otherwise, the dyes in the "putty" can bleed through your color coat and it will show when you are done. So, if you plan on using "Putty" (always use a putty that requires a hardener..never use the types that "air-dry"..they are junk for anything but woodworking), make sure you pick up a Quart of any type of Sealer that you like. This will be the last coat ontop of your bodywork before spraying your base coat (color coat).
When I start on a bike that already has a good paintjob on it (no flaking, existing paint doesn't appear to be "falling off" in anyway), I just scuff this paint with 280 grit wet/dry paper and use this as my "pallette". I dont worry about taking all of the existing paint off, and if the peice had stickers..AND they were UNDER the clear (you can't use a knife to peel them off without cutting through the clearcoat), I dont worry about taking them off. IF the stickers were on TOP of the clear, you'll need to remove them first, and all of the glue residue that remains.
So, I've removed all of the stickers, used "Goo-Gone" or similar adhesive remover to remove the glue, then sanded the entire peice down with 280 grit wet/dry sandpaper (VERY WET). After I think I'm done sanding, I dry the peice off and look for any shiny areas...if there are shiny ares, that means that area was not sanded..the entire peice must look scuffed...dull...you'll know what I mean when you see it. If there is an area that you may damage with the sandpaper,or is difficult to get into, you can use Red Scotchbrite pads to scuff these areas. I dont use it for the main parts of the bike, but in really hard to sand areas, or delicate areas, it works well.
After sanding, you need to spray a good thick layer of Epoxy Primer or Polyester. Follow the spec sheet on mixing this, and on drying time. DO NOT try to sand it early. It may LOOK hard, but the underlying layers may still be mush and you'll only have to start over. Sometimes, on thick coats...I let the peice dry for 15 hours or more.
After this had dried. Use 320 or 400 grit sandpaper and sand down any runs that you may have put in it, and give the entire peice a quick "once over", just knocking the bumps and nicks off. You'll see what I mean.
After you've knocked it down....get yourself a can of black spraypaint. You want to "FOG" the entire peice. You dont need to paint it black..just hold the can back about a foot, and "FOG" the peice. What you are doing is putting a "Guide Coat" of black over the grey (or whatever color your primer is). Now, put a peice of sanding foam or something semi-rigid in the middle of your folded wet/dry sandpaper, and sand all of the black off. What this will do is show you the high and low spots that may need bodywork. If you sand and sand in one area, but the black spot wont go away...that's a low spot. It will either need to be sanded until the black dot goes away (hoping you dont sand through the Primer first), or you'll need to add more Primer or Putty in that area. If you see a ring of black that wont sand away..there is a high spot in the middle of the ring. You'll need to keep sanding it down until the black goes away, or until you go through the Primer..or use putty to bring the black areas "up" to meet the high spot.
After all of the bodywork is done...if you didn't use any "Putty", you can scuff the entire surface with 320 grit one more time, dry it, and use a cleaner/degreaser to get all of your finger prints and such off....you're ready for paint (finally). If you used putty, you'll need to shoot a coat of Sealer, wait for it to dry, then scuff IT with the 320 grit...degrease and get ready to spray your color coat.
When you spray your color...read the spec sheet on air pressure, flash times, time between coats, and time to clear. You dont have to put it on in one coat. Usually, I "fog" three to four coats on before I see complete coverage. If you shoot a super-thick coat (like you did with spray cans when you were a kid), you'll only extend the drying time by about a half day (instead of 10 to 12 minutes) and end up with that will have to be sanded out. Many dry, thin coats in the color coat are MUCH better than one thick coat. Also...the color coat of a BC/CC paint job IS NOT SHINY!! This stuff dries like concrete...its dull, coarse, and wont look the "final color" when you spray it. Trust in the paint, and plug on, with dry, even, "Fogged" coats. After you've got a solid color...put two more "fogged coats" on. If you're shooting a pearl or metallic (ask the paint rep whether there are metallics or pearl in the mix), after you've shot these last two "insurance" coats...over reduce the color paint by about 10%, turn your airpressure up about 3 to 5 pounds from what you've been spraying it, and put on two more VERY VERY DRY, COATS. These coats are going to evenly dispurse the pearl or metallic on the top layers. You shouldn't need more than about a quart of your color. Mixed correctly, a Quart of paint makes a half a gallon of Sprayable Material.
If you've sprayed any metallics or pearls, DO NOT touch the peice's surface. You cannot wipe on these peices, as you will disturb the metallic particles. If you've sprayed a solid color, you dont NEED to, but if you think there may have been some oils that got into your gun when spraying, you can wipe down the solid colors with a semi-damp cloth with cleaner/degreaser and tack-wrag them before spraying your clear coats.
Your clear coat is the final coats that give the paint it's shiny appearance, and seal off the soft-easily scratched base coat. The clearcoat has some very nasty things in it...and although you SHOULD have been wearing a charcoal mask during the entire process..if you can only borrow one for one stage..this is the stage. Clearcoat can cause all kinds of nifty cancers that you dont want, and can cause death even from one time spraying. Wear a mask at the least! A full body suit and air-fed respirator would be better, but for backyard painters, it's just unrealistic.
Follow the spec sheets on the clear coat. Dont worry about putting on a super-thick coat and making it shiny on the first pass. I personally dont see a solid, shiny-coat until my third coat of clear. The first two are very dry, fog coats. I put on 4 to 6 coats, fogged of clear, then for my last two coats, I over-reduce the clear by about 5% and spray with a little more airpressure and a bit more distance from the peice. if a peice of dirt, fly, kid sister gets in the clear..dont worry. It'll sand out.
Let the clear dry as long as the spec sheet says. I personally dont touch the clear for at least 12 hours. Then, I start the sanding/polishing processes.
If you have any runs, sags, or bugs in the clear, work on those areas first. Get them flat using 800 Grit Wet sandpaper. Put a couple of drops of good dishwashing detergent in th bucket, this will help keep your sandpaper from sticking. After you've gotten those areas sanded flat, sand the entire peice with 800 grit paper. After sanding, if I think it's done, I dry the peice off and look for shiney areas. if there are shiny spots, (even tiny little specs of shiny-ness), I go again with the 800. After the entire peice is dull with 800, I get some 1000 grit, and sand the entire peice really well, then I use 1500 grit, then 2000 grit. The balance here is that you want to sand all of the peice flat, without sanding THROUGH the clearcoat. If you sand through, you have to re-spray the clear, costing time and materials. Try to not sand through.
After you've got the whole thing done with 2000 grit sandpaper, you need to use a polishing system to make it "shiny" again. I use 3M's Perfect-It II compound/polish system. You can get the materials at any O'Reilly's auto store, or at most paint-suppliers. Doing a peice by hand is a very labor-intensive job, but it CAN be done. Using a buffer is a much better way to do the polishing, but, again..you have the ability to ruin the entire thing if you buff through the clear.
If you've made it this far, and you've not sanded or buffed through anything...congratulations. I hope everyone attempts this at least once. It a very difficult process that takes time, mechanical ability, artistic ability, understanding of materials and quite a bit of money. But it can be fun and make your bike stand out from everyone elses if done right. There are many books out on custom painting, and just because it doesn't say motorcycle on it, doesn't mean the processes inside dont apply. $25 at Barnes and Noble may save you $400 at the paint shop if it can keep you from screwing up once or twice.
Afte all this typing..I gotta put in a shameless plug for my custom paint shop. I run Grasshopper Custom Paint. We've been doing bikes, street rods, helmets and custom stuff for the better part of 10 years. You can see some of the stuff on my outdated website at www.grasshoppercustompaint.com
Anyways, hope this helps. If anyone needs more help, PM me.