Using Epoxy

There's always a big question of what to do with bare metal when restoring a car.  One of the options is to seal it with epoxy primer.  Epoxy is can be a very good primer that does a great job adhering to bare metal and sealing out moisture.  There are a few things to know about using epoxy to help ensure it works as good as its supposed to.

Metal Prep:  The metal needs to be completely clean and extremely well prepped.  This means any rust pits, no matter how small, need to be media blasted until the metal at the bottom of the pit is completely white and clean. You can, and should, use a bright light up close to make sure it's actually clean.  The metal must be cleaned extremely well with wax and grease remover prior to any sanding or blasting.  The metal that isn't blasted needs to be cleaned, then d/a sanded with 150 grit, and then 80 grit.  The finer grit first is to more thoroughly sand and clean the metal.  The 80 grit is for better adhesion.  After sanding, the metal must be cleaned with wax and grease remover until the towels are completely clean.  Then, spray your epoxy.

Dry Time:  It cannot be stressed enough how important this step is.  Epoxy is a very slow drying product.  It requires, preferably, at least 2 full days to dry before anything goes on top of it.  Preferably it should be baked as well, a lot.  If the epoxy is not dry, there is a good chance that it will continue to dry under your body filler or primer, and cause it to delaminate.  This doesn't always happen, but it does happen, and is not worth it.  This is especially true under polyester primer.  Be very aware and very careful about how heavy any epoxy is sprayed and how dry it is under polyester primer.  2-3 medium coats would be normal when sealing bare metal panels to be body worked.

Prepping:  The epoxy is fully dry, now what?  The first step is to use a scotch brite pad and scuff it.  This first step is a way to make sure that everything is at least scuffed, incase anything is missed sanding it.  Once it is scuffed, it can be sanded with about 120 grit.  This can be done carefully with a d/a sander, or by hand.  Try very hard to not sand through the epoxy.  Then, blow it off good and clean very good with wax and grease remover.

Under Polyester:  Something to consider and be aware of when epoxying bare metal areas prior to polyester.  It is typical that body filler will be finished in 120-150 grit.  That is a pretty aggressive scratch for epoxy to be in, considering it's something that dries slow and shrinks.  Once the epoxy has fully dried, there will be epoxy at the bottom of those sand scratches that isn't sanded or prepped.  The danger is that it might still be too wet in those scratches, or it just won't be prepped.  The point of polyester primer is it doesn't shrink, and is a very good solid primer, and having that epoxy under it possibly defeats that purpose.  

Finally: High quality epoxy can be a very good way to seal metal and a great foundation for body work to be on top of.  It's very important to be aware of the potential problems with it, like the dry time.  Most tech sheets do not say to let it dry that long, and that is highly misleading, and a huge gamble to do all of your work on top of.  Also, consider that when prepping the epoxy, you will sand through to some metal.  During body work and all the block sanding, you will also sand through the epoxy in areas.  Remember that epoxy isn't a magic answer, it is absolutely always about how well your metal is prepped.