hello,,,ive got my garage doors to paint soon when the weather gets a little better and was wondering about undercoats,,,last year I did some of the smaller doors on the property and used white undercoat which then took a few coats of black to cover the white,,,should I be using a dark undercoat???,,,Ive never seen dark undercoat when ive been buying paint before,,,what do you lads in the know suggest???