Surface prep

1) Clean, sound substrate
2) Quality products
3) Patience

Everything else can be relative.

I've never used that particular product but I have used similar ones on the past. I have looked at the manufacturer's spec sheet here. It appears it is an old technology, lacquer-based product that is marketed as a dual purpose primer and surfacer combined. If you go over the spec sheet carefully, you will notice for the best adhesion and corrosion resistance, the manufacturer recommends applying a dedicated etch primer under it on bare metal. Or keep the direct application to bare metal to very small areas. This is because this type of product is not the best choice when applied directly to bare metal. That's true for all similar lacquer-based primer products, not just the Dura Build.

I'll give you my own thoughts. The choice of any combination primer-surfacer product is not the best choice for bare metal, especially the relatively rough surfaces of cast and forged products. A surfacer product is intended for filling of minor sandscratches, etc and is intended to be sanded after application. Your axle is probably forged and your engine block is cast. Neither is smooth like sheet metal, nor are they intended to be. Unless you want to spend an awful lot of time sanding, a surfacer is not even the correct product for the application. Attempting to make cast and forged components have a mirror finish is a waste of time as far as I am concerned. Plus it will not look "right". Save that effort for the sheet metal parts.

The best choice of primer for cast or forged products (or any bare metal for that matter) is a non-sanding product that has excellent adhesion and corrosion resistance. That's a two-part automotive epoxy. That's my opinion and it's what I use exclusively. Some will argue that an etch primer is just as good, but I'm not one of them. An etch primer is most always recommended to be followed by a surfacer or other primer prior to applying the topcoat. That leaves the application of two separate products before the color. And with the surfacer, you should plan on a lot of sanding. That can be avoided by the use of a non-sanding epoxy with the color applied directly thereafter. No sanding required. And you will have the very best adhesion and corrosion resistance. You need to be aware of the time window for topcoating the epoxy however.

I am not going to advise you to remove the Dura Build. That's your decision. You have time, effort and money already expended with it. I believe it will work as it is, but it's not as good as if the choice had been an epoxy instead. I think there are only two reasons to use any lacquer-based primer-surfacer product given today's choice of better materials: 1) they're inexpensive and 2) if a surfacer is needed or desired, they do not require isocyanate-containing additives and therefore can be safely used with a standard cartridge respirator instead of a supplied-air arrangement.

Rod

"You need to be aware of the time window for topcoating the epoxy however." Does this mean the final paint color? In other words, do you have to paint it shortly after primer? Thanks, Doug

No, the epoxy is a primer only. It has no filling ability to any appreciable extent, although two coats will fill a typical white metal blast profile. It's not for filling sandscratches (at least those obtained with the coarser grits) or other imperfections. For that you want some kind of surfacer product. Most automotive surfacers have filling ability up to a few mills deep, usually with several coats and a quite a bit of sanding. They are intended to be sanded so they sand quite easily. In fact, most of what is applied will wind up being sanded off in the process of getting a smooth surface. If the depth of imperfections is more than just a few thousandths, then different measures are probably called for. One choice would be a sprayable polyester (sprayable bondo) but that usually requires a larger tip and needle size for serious application. Another choice would be a spreadable polyester finishing putty. No matter what choice you make, there will be a lot of sanding involved. And for many tractor parts, that's probably hand sanding. A mini 2 or 3 inch pneumatic RO sander can be put to some use if you have one. There will still be a lot of hand work however. For sheet metal, filling rust pits is a real PITA, using the spreadable finishing putty, but the result is generally worth the trouble. To approach typical castings that way is not my kind of tea. You'd have to want to do it real bad and I don't think it would look right when done, but hey, to each his own. If mirror-finish castings are desired, it can be done if you want to invest the effort.

As far as the epoxy time window goes, yes, there is a time period (window) after application of the epoxy during which the topcoat should be applied. That period varies with the epoxy brand and exact product. I've seen it as short as 24 hours and as long as 7 days. The PPG OMNI MP170 epoxy that is mentioned a lot on this forum has a window of 3 days. The purpose for that is to obtain a good chemical bond between the epoxy and the topcoat. If you fail to meet that window, most epoxies recommend you then scuff sand, say with a Scotch-Brite pad, and re-apply a coat of epoxy prior to proceeding to your color topcoat (or surfacer product, if used) within the newly established window period. The reason for that is to obtain the best adhesion of the topcoat because the topcoat will no longer chemically bond with the epoxy after the window has passed. The epoxy will have cured too long for that to happen.

Rod

Epoxy primer seems to be the sh*t. I LOVE the fact you don't have to scuff the primer to paint topcoat on.(within window of course)

Thanks for the info and your time Rod. Doug

Rod