Like Drew was saying, If your dings are shallow you might be able to shoot a couple of coats of surface primer, (It's a build up type of primer),then spray a guide coat, (contrasting color to you primer),and block sand with a long board. If you use a black guide coat on a grey primer and you see dark patches, those are low spots. If you see those you need to prime,guide coat, and block sand again until it's flat. I painted my car black, and it took months to get it straight! Be prepared.
------------------
http://members.tripod.com/~SnowSurfer/mikekz1.html