If you want to make your own filler, use this recipe 250 ml of resin w/2.5ml of hardener, 200 ml Glass bubbles, 50 ml cabosil, 30ml of surfacing wax.
This will yield a cup and a half of strong but easily sanded filler. You'll have about 25 minutes before it kicks. That's why I only mix a small amount and then make more if needed. If the spot isn't to big then filler works fine. If not, then go with PB and then come back with the filler to fill any low spots in the PB. Sand and fair with 120 grit and then finish with 180. You want some fine etching to give the primer something to bite into.
I'd also recommend 3 coats of the final Color.