Basic primer question

You may have misunderstood. My point in posting about the gas cap chip was to note that the separation was not at the topcoat/epoxy interface but went all the way to bare metal. The fact that the topcoat/epoxy chemical bond held up in that particular instance is likely a validation of your practice of using epoxy as a sealer over surfacer to obtain a chemical bond with the topcoat. A previous chip problem that I had with the same paint was at the hand crank opening in the radiator shroud. There was enough slop/wear in the crank guide structure that the crank actually rubbed at times on the side of the radiator shroud and a chip got produced at the edge of the opening. Since it was caused by a fairly high concentrated bearing force on a rounded edge area, there is probably no paint system that would have held up there either. As I remember the separation point there was the topcoat/surfacer interface since I used surfacer on the shroud and it had epoxy under it. That would also validate the superiority of a chemical bond over a mechanical one. I am thinking about using your epoxy-sealing method whenever I use a surfacer. Would you guess that a chip separation point would occur at the epoxy sealer/surfacer interface (a mechnical bond point) rather than going all the way to bare metal, when using the following layer sequence?:

1. Two coats of epoxy direct to "bright metal" sandblasted steel - no chemical pretreatment.
2. Several coats of 2K surfacer applied within the epoxy window.
3. One thinned "sealer" coat of epoxy after sanding the surfacer (say with 400 grit final).
4. Topcoat within the epoxy window.

I am not asking for any degree of certainty here. That's not possible. Just your expectation under that scenario. What I would want to avoid is to have a chip (if it's going to happen anyway) go all the way to bare metal. Those first two coats of epoxy provide an excellent barrier protection against corrosion for an indefinite period. From that standpoint I would prefer to have any coating adhesion failure above them. I am assuming here that the bond between the first epoxy and the sandblasted profile is likely stronger than the bond between the sealer epoxy and the 400 grit surfacer scratch, but I don't know.

BTW, no can of worms and that wasn't a test. I had fumbled the cap after taking it off and it dropped to the floor. Here it is from a previous post:

Rod

If most of your parts are blasted to bright metal, you have the very best foundation for any coating system. In my opinion that is deserving of the very best bare metal primer - an epoxy. It doesn't have to be an expensive premium version but it should be an automotive, two part epoxy. In some cases, if you don't have to do any heavy sanding or other type of bodywork, you can skip the surfacer altogether and topcoat directly to the epoxy. Although it's not a filler and is basically non-sanding (although it can be sanded) it will fill a typical blast profile quite nicely with two coats. Therefore a surfacer product can perhaps be eliminated. Of course you probably would not be using a surfacer on the cast parts anyway. I think you have indicated previously that you have fresh air breathing equipment? Assuming you do, then I would loose the lacquer-based primers/surfacers altogether. Either keep 'em around as a monument to times past or give 'em to someone who doesn't have the proper breathing equipment to safely use better stuff. Get some 2K, urethane-based surfacer to go with the epoxy for when you need to fill sandscratches, etc. Like the epoxy, it doesn't have to be an expensive item from a premium line. Those two basic products will provide you an excellent undercoat system for about any type of topcoat you wish to apply. I have never used Martin Senour products and don't know enough to talk intelligently about them. I do know that they have more than one "quality" (and price) line of automotive paints, just like DuPont, PPG and others. I am sure they have similar products also, like epoxy and 2K surfacers. I know that their stuff has a following around my area because the local NAPA store seems to be doing well selling it - even with both DuPont and PPG competition within five or six miles. We could argue all day about whether you should go with a premium product or an economy product. Only you can judge how much you want to spend for what may be unimportant (to you) marginal differences between premium and economy qualities. The cost difference can easily be two or three times between the two. The bottom line is a two-part epoxy and a 2K surfacer in any line of automotive paints from a recognized manufacturer should serve you very well indeed. As CNKS indicates, you need to be aware of epoxy recoat time windows to obtain the best performance. That will be spelled out in the appropriate tech sheet. If you don't have a link to the MS website that has both technical data sheets and MSDS sheets for all their products, post back.

Rod

Rod

Rod