i have built a home sub for my room out of mdf. I just painted the box with fiberglass to seal and to avoid the mdf from soaking up the paint.
I plan on painting it a shiny black using krylon primer, black lacquer paint, and clear coat.

I need some kind of filler (not bondo something lighter then that) to fill small pin holes and scratches. I read that you should thin out the filler and hand rub the entire box with the filler.
I would say the filler is really the only thing I am unsure about here so some specific recomendations would be helpfull.
Also how do you thin it out filler?