Just any kind of rattle can primer that can be wet sanded will work fine. Honestly, the priming is the most important step in the whole deal, the paint will only turn out as good as your primer. I will list my steps for you...
1) Sand the fenders with 150 till they are kinda harryish
2) Fix any cracks and stuff you have (plastic epoxy is great for that)
3) Prime then wet sand with 600
4) Prime again and wet sand with 800
5) I primed again and sanded with 1000-and again with 1200 but you dont have to, I just wanted a glass like finish on my paint
6) Paint thin even coats of Fusion
Here they are sanded down to 150
Here they are after all the wet sanding stages
Here is the front end done in Krylon Fusion, waiting for clear
Hope this helps you out! Let me know how it all turns out, good luck :thumbsup: