iwannadie.... here's a primer on painting outside. i did my bike and am very proud of me work.
1) while painting outside, dont be under any trees, preferably a backyard/frontyard with a lawn, so that dust and s**t doesnt fly up and stick to your paint job.
2) you can *sandblast* plastic parts do not worry I did so. That is the best way to get the paint off them without altering the geometry of the parts, and you *need* to get the old layer of paint off. You can probably barter something for a quick sandblasting job... or the local college machine shop usually has a sandblaster where you can DIY it, provided you know just about any student going to school there. you can even do the same to your tank, but make sure all the orifices to the tank are sealed. or if you get sand in there, prepare to wash it out with water and then cleaning alcohol to get rid of the water
3) Prime the completely sanded parts. You may use adhesion promoter for the plastic parts, but thats not really necessary. Apply my multiple light coats of primer and let them dry between coats. Same with the paint.... for a good durable finish you should apply at least 2-3 thin coats, and let them dry to a tacky finish between coats. In fact outdoors you probably reduce your drying time.
4) if you're paiting outdoors there probabaly a lor of crap folating in the air. one w ay to prevent it from sticking is to keep fans around the tank to blow the stuff away. remember to staple a bedsheet or something to the opposite side of the fan so it doesnt suck any crud in. I painted my tank white too,, outdoors, and not a speck of dirt on it

. good luck.