Well Dave I was going to put my $.02 worth in but I believe all the bases have been covered with very good sound body work advice, Randy did mention using two colors of primer, it's called a guide coat in the body shop world, I use this method when working with low spots, I prime and then when primer is dry go back with a dark color I just use some cheap Walmart $1.99 black spray paint and put a very very light mist coat on, it won't take much, let dry and when you start sanding the low spot will remain dark it will stick out like a sore thumb, and remember what Dale said PATIENCE!! hope this helps, hang in there!