Which comes first...Primer or Filler?
A question has come up this week regarding whether you should apply filler first and then epoxy or vice versa. Every body shop has its own way of doing things so I did some research, to see if I could set matters straight. Please read the following and then you be the judge.
Autobody filler is (basically) made of a vinyl ester or a polyester type resin. Epoxy primer is (basically) made of an epoxy type resin. If you were to bond/glass over a metal pad to a fiberglass boat, vinyl ester or polyester is never used, epoxy is. There is no way a plastic resin can make a chemical bond to metal, it's going to be a physical bond. Epoxy has better flow characteristics to bond with those microscopic imperfections and bite in. Now that the metal has a bonded resin coating, the vinyl ester will stick much better to the epoxy than the steel, especially if the epoxy hasn't totally cured (7 days or so). If you are bored one day, do a test patch on some scrap sheet metal. Try both methods...metal-filler-primer vs. metal-primer-filler. Take a hammer and see which one sticks better.
A body shop I deal with had an early '70's Mustang come in after an accident. The car had been repaired and repainted at another shop years before. There was body filler all over the bare metal of the hood, underneath primer, followed by paint. From the accident, the filler (a skim coat really), all came off exposing tons of surface rust on the metal. It had been slowly growing under there for years.
Years ago, the accepted method was metal-filler-primer but with new technology, new products and as we learned from past mistakes we have found that the most effective and durable way to go is metal-primer-filler. This takes care of any microscopic rust, bonds far better and lasts much longer, in our professional opinion.