Alexideways has the right idea, it's called guidecoating. I lay down a couple coats of a high build primer and then mist it with a contrasting color. You will see the dents as the board won't remove the paint in the low sport. Get use to using your hand to assess the flatness too, rub it back and forth over the area in question and with a little practice you can get pretty good at finding the highs/lows. When you sand always use the longest sanding board possible, I use an assortment of different length boards along with wooden dowels, rubber hose, etc. When your sanding sand in an X pattern. Yep, you can expect to put a lot of hours in to it to get it straight. Remember, with body work you only see what you did wrong when it's finished