I used POR-15 on my last truck and Zero Rust on my present truck. I used the cheap foam brushes for both because as mentioned before, the brushes are toast after one use. I had a couple different sizes on hand for those tough to reach spots and the wider (1 or 1-1/2") brushes for those wide-open-road-cruising spots, like the outside of the frame.
The directions call for two coats of POR-15/ZR and then a top coat. I only put the top coat (spray can gloss black) on places that would see the light of day. When I sold the other truck 4 years later, it still looked as good as the day I finished it.
After completing my present truck frame, I found that the POR-15 smoothed out better than the ZR after being brushed on. The benefit of ZR is it comes in spray cans. On the next truck (don't tell my wife there's going to be another one), I'll use ZR again but only in the spray can.
