I used POR-15
works wonderful...it locks in the rust very well....
one thing to note is that when you apply the POR-15 stuff, it leaves a brown coat to lock in the rust and if you dont paint over this coat within say 2 months or so, it will start to peel away....
My point is that you need to paint right after the stuff dries...
I used it a few places on my Toyota Truck and it did well...they have starter kits too with all the supplies that you need for a small area of a car for like $15, so the price is perfect...that is the kit I bought and there is more than enough POR-15 to do touch ups with....
for small touch-up, it might be worth it to use the touch-up paint...if there is a good size spot of ruse, it might be worth it to put down this POR-15 stuff first!!!!