I've asked Daron to add his comments to this thread. Hopefully he will
.
180OUT, I used to do what your friend did (was even taught so by my brother inlaw) ... and one night I was having a beer with a mate (who I used to race against) and he owns one of the top panel shops in New Zealand and is Rolls Royce approved, etc. and he said:
"The most common mistake amatures make is not sealing the metal before filler and paint, the steel then sweats and before long they get rust again ... in the same places they just fixed"
Now he might have been drunk
, but I've also watched Daron's threads and he makes a big deal about sealing bare metal ... leaving cars for as long as possible to let the sealer sink right in.
Pete
ps: Remember really old school guys were taught leading, and that is a bare metal process ... then filler came out and I believe many incorrectly applied it like it was lead .... It's not, it really should be thought about as another layer of paint.