Picklex is a very good product. I used Crud Cutter I bought at ACE hardware. As long as it has phophate, it'll work.
You can use a spray squeeze bottle that does a heavy mist to avoid runs. For areas where rust was heaviest, I recommend a double dousing of the area. Make sure to wear rubber gloves and eye protection. Phophate is very harsh on skin. after you've let it sit and flash dry for 15-20 minutes, rinse with water.
As it dries, and you'll begin to see a white chalkiness form on the metal. Right before you're ready to Epoxy prime, take a 3M scuff pad and lightly scuff off any excess build up that was left from the phophate dried on.
Now your metal is squeeky clean ready for primer to make adhesion into the metal pores.
Here is a more a detailed info on the process. It gets a little scientific about it.
http://www.pfonline.com/articles/pretreatment-for-paintingAnd a thread doing it...
http://www.autobody101.com/forums/viewtopic.php?f=5&t=2774