Here's the problem with debates like this... IMHO.... and everyone on here knows I'm the "H" part of that...

Someone that has ALWAYS done something wrong... will debate endlessly "why" they do it wrong. In the end - what we're all looking for is the PROOF of LONG TERM results.

In that vein --- I would think that the right way to do this is to see what the actual manufacturers have to say about their product usage.


Here is the text copied directly from 3M "tech specs" for their brand AKA: BONDO

• Clean surface with warm, soapy water to remove all wax, grease and oil. Allow to dry. • Sand the damaged area with coarse 80 grit sandpaper to remove all paint, primer and rust down to bare metal. Sand 1 to 2 inches
beyond damaged area. Remove sanding dust.


EVERCOAT -- Rage Gold:


Since I can't cut and paste their FAQ on this topic I'll just tell you basically what they say.

They want their product applied to CLEAN BARE METAL -- Unless you want to apply it over epoxy primer.... Which is 'okay' IF the metal manufacture requires that.. but they'd prefer just clean bare metal.

Here's the link to their (Evercoat) website with FAQ's discussing this application:


http://www.evercoat.com/faq.aspx

If I recall correctly the DP90LF has a 7 day window where the filler will stick to it just fine....just as good as metal. If you go past that window, you need to scuff it with 80 grit before applying filler.

As for the moisture debate I think the issue with is that alot of guys will do filler to bare metal but then it will sit for quite awhile before they epoxy over it....if it was epoxied shortly after filller, it would seal the moisture out just fine....its that gap of time that is the issue

This can be debated forever.... there will always be guys that insist on doing filler to bare metal, and have been doing it for 40 years and never had a problem.....and then there will be the guys that are more flexible as technology changes and are willing to change their techniques as the products change.

My advice....follow the tech sheets....

Blake-

What filler are you using?

Do fillers differ in moisture absorption over the short term and how they react to bare metal vs epoxy applications?

we have always used Evercoat fillers, and the Slicksand spray from Evercoat is Awesome, spray it on like a filler primer, setus up fast 2-3 hours sands fast and next to ZERO shrinkage. you can leave it in the mixing cup and even after a month it is a perfect mold of the cup. unlike most filler primers that you have to add reducer to. you can coat the whole car and long board it down to 220 and then a thin coat of final prime, fian sand and paint.

as to the second ? no idea that is a ? for the chemists.