This is WAY out of line. Men, reading INSTRUCTIONS??? What's next?? Cats living with dogs?? Santa Claus at Christmas?? We must ban this kind of reasoning. Cosmo
I always clean the bare metal and seal it up with 2-3 coats of a quality epoxy primer. Let dry a day or two, scuff the surface with 180 grit. Blow the dust off, then wipe that down with a wax and grease remover. Let any trapped solvent, ie wash and wipe, or wax and grease remover evaporate for 15 minutes. Wipe the panel with a tack cloth just before appling polyester filler. Either way you choose to do it, cleanliness is so important. A layer of dust/ dirt can ruin adhesion either method.
I've seen this question before, and forget the bondo, asking is like opening a can of worms. I'd let the car be the judge, if it needs a lot of smoothing then maybe do the filler first, if it's pretty clean, then epoxy first.
Don't think of it as learning. Think of it as cheating, like looking up the answers in the back of the book.
I'm not voicing an opinion 1 way or the other but when I got out of high school I built a 38' Willys pickup and did the usual door bottoms/lower cowl patches etc. Filler was put over properly prepped bare metal and the truck stayed with me for 12 yrs after it was done. The majority of that that time it sat outside as I had no garage space at the time. Even through Michigan winters and humid summers the truck looked just like I painted it the day before with -0- breakouts.
When I was stripping down my 58 GMC I found evidence of both approaches from over the years and was convinced of Epoxy first when I found a rust pocket under a direct to metal application. It was clearly from the trapped moisture as there was clean metal on the perimeter of the 'patch' and rust at the center. I think it's good to consider that primer wasn't a sealer 'back then' so nothing was gained by primer first, but now with epoxy primers ability to seal out water it makes sense... to me.
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