The trick to finding the low spots is you prime it then spray flat black spray paint. Then you block sand the black spray paint off. Where you have dark spots those are your low spots. Then you fill em with some filler spray the black again and block sand again. You repeat the process until all the low spots are gone.

Can't see the pics at work but I could picture it in flat black with some red pinstripping with red powdercoated stealies and baby moons.

I believe shows like overhaulin skip that step because it is time consuming but it's well worth it in the end.