Jerry I tend towards 'hoping' the modern higher quality polyester fillers (bondo) have less talc and more high tech fillers like ceramic spheres and the like, so they are less absorptive.
My understanding is it's the talc that is the problem.
Great Bondo knows it was a lot of white filler I kept finding on my truck... and what the rust was under.
But I 'Hope' for better corporate citizens too...
So I attribute the particular spot of rust I had in mind to Roberts scenario of 'Hot Mix'.
DavidF's reasons are spot on in my book.
Soon as it's clean it's rolled brushed or sprayed.
I'm presently trying out a Black epoxy 'temporary' paint job on my 1/2 OT truck. My plan is to see how it sands down for sealer and a top coat in a year or so.
And just for perspective... The rust under filler, I'm talking about a dime size chunk of rust under a 25+ year old repair that was little more than 'surface rust'. So NOT the end of life on earth...
NOTHING like the Whole holes caused from plain old dirt caught in cab corners or cowls.
