The lite variety of dry wall mud (joint compound) has worked well for years as a simple and dirt cheap filler and it thins with water. Fools you when applied and seems heavy but when totally dry and sanded its as good as any specific packaged product for our purposes.

Steaming dents out talked about above is a neat trick that works well some kinds of dents and depressions. Along the same line a dent or shallow grove on an already covered surface (film) can be made frequently invisable by peppering the dent with tiny holes made by a dressmakers pin or the tip of a #11 blade and just puddling up a little water over the dent then running a covering iron over and heating.

John