For the spots that you can't get to the back side with a dolly, a pin welder and pulller can get the low spots pulled back up. If you're not willing to invest in the tools and learn to use them, it may be better to take it to a body shop that still does metal working as opposed to slapping filler on. IMO, body filler for more than minor imperfections is a route to a result you won't be happy with.

Search for some Posts by MPandC and do some reading on metal repair.


Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.