After i had sanded back the whole backbox, it was time for the 1st coats of undercoat. I masked off the aluminium grounding inside the backbox, and after dusting it off gave it a couple of coats of undercoat so i could see what other defects i had to repair. The worst of it is, of course the side with the planking, so after this has dried i will sand this back and give this side extra coats to hopefully fill the cracks, it should work, as its looking much better already.
Tomorrow, if the paint has dried enough, i'll sand it back and fill any imperfections with Polyfilla or spot putty as needed.. oh and drill the hole for the backglass lock which i forgot to do before spraying 
Have you tried Solver Maxi PSU?
It is water based and when you apply it with a 100mm foam roller it fills minor imperfections really well, it dries fast and is easy to sand.
It is not necessary to remove all the old paint as it sticks to any surface.
