Kevin,
I recently started painting my own canopies. It's not a quickie wam-bam thing. If you want a paint job that won't get attacked by the exhaust/fuel and turn into a gooey mess you MUST use fuel resistant paints. The only paints that can with stand this are automotive quality paints. Urethanes, Lacquers, etc. are about all that can hold up(and not the Lacquers that you can buy in a spray can!).

If you want your paint job to stand up, follow Joe's tips for prepping the plastic by sanding and using an adhesion promoter. After you have primed and sanded and primed and sanded you will have a decent enough surface to paint. You can use Rustoleum or whatever rattle paint you want but it must FULLY cure between each color. That usually takes at 2-3 days. Then if you don't have painting equipment to shoot a an automotive clear coat, I would just take your canopy(mounted and prepped to be shot) to a local body shop and ask them to shoot a couple of coats of clear on it when they do their next car. If they charge you anything it shouldn't be more than $20.

Give it a try. Paining these canopies is a hobby of it's own. It is VERY rewarding and alot of fun. Although, nobody should bother doing it on their own unless they just simply like to paint. It takes you quite sometime to recoop your $$$ if you have to go out and buy a compressor, gun, supplies, respirator(sp?),......