Finally, some progress. Started by a rough sand down on some drywall mud that I had already applied, leaving it ready for the next layer of drywall mud to reach a near perfect surface. Then prepared some more drywall and applied it on the left and right side of car, covering almost all the both sides, leaving only the "gills" with out being, since I run out of drywall mud... The first drywall mud that I was using was from "Pladur", but when it run out, I used one from "Robbialac"which was laying around. As it turned out, the one from "Robbialac"is easier to apply, and I think it drys more quickly. Now I don't know which one should I buy, the "Pladur" that is much much cheaper or the "Robbialac"...