You are asking for failure. Preparation prior to finishing is 99% of the job and it seems that you want to take some short cuts.
Don't.
If you really don't want to take the time (or don't have the mechanical ability to do a proper job) then I suggest you either have someone else do it or just forget it.
Here's how to do it right, but you MUST follow the directions... no shortcuts!
Painting a bike well takes a bit more than a whim, a 6 pack, and a weekend. Do it right and it will look great, do it wrong and you'll wish you never did it.
This answer is long but will give you an idea of what to do. Want a quick answer? Take the bike to an auto body shop and have them do it. It'll cost about $100 if YOU take the bike apart.
Use DUPLICOLOR brand LACQUER available at most auto parts stores. In the UK, I don't know if the brand exists so you'll need to locate a brand of acrylic lacquer. DO NOT BUY ENAMEL OR URETHANE!
You'll need 1 BIG can of grey sandable primer , 1 can of base coat (if the color requires it... many do... read the can), 1 BIG can or 2 SMALL cans of the color you want, and 2 or 3 BIG cans of clear LACQUER.
Here is a quick explanation of what to do...
1) After completely disassembling the bike have it sandblasted to remove ALL the finish. If the frame is aluminum, it should be walnut shell blasted, or you can chemically strip the paint. Just make sure all the paint is OFF the bike.
2) READ THE CAN INSTRUCTIONS, then spray it as soon as possible with grey SANDABLE lacquer primer. DUPLICOLOR brand paint is all lacquer, I think, and you can get it from your local autoparts store. Buy the BIG cans, not the small ones. Apply at least 1 full can, in coats, 15 minutes apart. Don't worry about light runs for now.
3) Wait at least 48 hours. There is a difference between dry paint and cured paint... it needs to be cured. Sand the primer using 220 grit aluminum oxide paper (white in color) or wet/dry paper (black in color) until the surface is smooth. It shouldn't take much unless you have a lot of runs.
4) Using a tack rag, lightly go over the entire frame (don't rub!) to remove the sanding dust.
5) Apply several coats of the base color (if necessary), then color of your choice. Make sure the frame is completely covered, it is not necessary for the color to be on thick. Follow the directions on the can. Again, use LACQUER, the same brand that you used for primer. Apply coats 1/2 hour apart. Be sure you cover every part and do NOT get runs. If you get runs you are applying it too heavily.
5a) IF you wish to add decals, wait at least 8 hours, apply the decals, then continue. Make sure that you avoid touching the new paint as much as possible and to make sure there are no marks from water or "sticky stuff" on the frame. If you don't want to add decals, continue immediately to the next step WITHIN 24 HOURS...
6) Do NOT sand the color coats. Apply the LACQUER clear coats, perhaps 2 full cans worth, coats 15 minutes apart. If you put decals on you should spray at least 4 cans of clear.
7) Let the frame alone until you can't smell the paint anymore when you get up close and take a big whiff. If you can smell it, it ain't dry! This will take 2 to 3 days at least.
8) Wet sand the entire frame CAREFULLY with 800 grit silicon carbide wet-or-dry paper (black in color), then 1000 grit silicon carbide wet-or-dry. Make sure to keep the paper wet and to wipe away the white slurry that forms. If you start to see the color in the slurry, STOP! You have sanded too hard and reached the color coats. Repeat steps 5, 6, and 7, then try again. Don't try to "touch up" the area, it will look terrible.
9) Using white paste polishing compound, follow the directions and polish out the whole frame.
10) Do NOT wax or wash for at least 10 days.
If you rush this you will end up with a sloppy mess and you'll kick yourself for ever trying. However, if you take your time and do it right it'll be a very cool feeling.
