I have had tremendous success with Evercoat "metalglaze" polyester finishing and blending putty #100416.
It is a 2-part putty that requires a hardner, and it can be applied over body filler or primer, (not bare steel). It dries quickly, sands very easy, and doesn't seem to shrink, even when applied fairly heavy ( like up to 1/16" thick).
You will need to seal it with primer before spraying finish coats of paint over it, or it will suck the shine out of the paint applied over top of it.