Bare Metal: Etch or Epoxy Primer? (By the Sheet)

Bare metal: etch or epoxy? Bare metal, and four cans on the shelf: etch, epoxy, high-build and sealer. Here's which one goes on the metal. Etch primer bites, and it's thin. Epoxy is the foundation: it's the one the sheets let body filler go over. And high-build and sealer come after, not instead. Four names, three jobs — and two of those jobs come out of the same can. All of it from the makers' own data sheets. Everything in it is at autofxmart.com, link below.

Three jobs, in order Read the sheets and there are only three jobs. One: stick to the metal and keep it from rusting. That's etch or epoxy, over bare metal sanded eighty to one-fifty, with the repair featheredged one-eighty to two-twenty. Two: fill the sand scratches and the low spots. That's a high-build primer — two to four mils of it, blocked back flat. Three: even the base out so the colour lays over one tone. That's a sealer, one full wet coat. Three jobs — and two of them, on these sheets, are the same can mixed two ways.

Etch or epoxy on bare metal? Start with what they leave behind. U-POL and High Teck both put their etch at fifteen microns, which is point six of a mil, in one or two coats. Two passes of SprayMax's two-K epoxy is two point eight. Both take colour in about thirty minutes, and the epoxy will still take it without sanding seventy-two hours later. Now the line that decides it. Transtar's epoxy sheet gives a time for body filler over it: one hour at seventy degrees, two at fifty, forty-five minutes at a hundred. Not one etch sheet we read names body filler at all. The claims differ too. Etch: adhesion, a barrier against rust — and U-POL's own etch sheet says if the repair needs build, use a primer filler. Epoxy: corrosion resistance, better filler adhesion, and water kept out. So: etch for a rub-through you're painting the same day. Epoxy when there's filler, bodywork, or any wait.

The etch primer sheet Here's U-POL's etch sheet. One to two coats, about ten inches off the panel. Three to five minutes between them. Fifteen microns of film. Then it takes colour directly: under normal circumstances, the sheet says, no sanding. If you do sand, four hundred to six hundred dry. Sixty-eight degrees, and shake the can two full minutes. Two years on the shelf, chromate free. SEM counts two to three light coats, only enough to hide, and colour in ten to twenty minutes. These are professional products: read your own can's sheet.

What 0.6 of a mil looks like Point six of a mil. Look at the sanding scratches. It didn't fill anything. That's the other primer's job.

The epoxy sheet Transtar's epoxy, then. One part primer to one part activator, no induction time: mix it and spray it. One to two medium wet coats, ten to fifteen minutes apart. Three to six tenths of a mil each. It sands at an hour. But here's the number that changes how a shop works: you can recoat it without sanding from thirty minutes to seventy-two hours. Epoxy a bare panel on Friday, come back Monday, carry on. Filler goes over it at an hour at seventy degrees — two hours if the shop is cold. Seventy-two hours of pot life. Five hundred and twenty square feet to the gallon. High Teck's epoxy is one to one too, and wants a scuff past forty-eight

The same can is two of the four hours. Now the part nobody reads out loud. That same Transtar can has two sets of directions on it. One to one is the primer, straight onto bare metal. One to one, plus ten percent urethane reducer, is the sealer. The sheet prints two sets of headings. As a primer: direct to bare metal, one to two medium wet coats. As a sealer, groundcoat or precoat: one full wet coat. Same can, same activator. High Teck does it too. Four to one is a high-build primer. Reduce that same mix ten to twenty-five percent and the sheet calls it the sealer — and the product's own name says both: two-K high build and sealer. Whichever way you get there, High Teck's sealer sands in sixty to ninety minutes and takes colour with no sanding for twenty-four hours, maximum. So sealer is often not a fourth can at all. It's the third one, thinned. Read the mixing box before you buy another gallon.

High build: the one that fills The high build. Four parts primer to one part activator. Sand what's under it one-fifty to two-twenty. Two to three wet coats. Two to four mils of dry film — four to six times what the etch left, and that's the point: it's there to bury sand scratches. An hour at seventy degrees, then block it flat, four hundred up to eight hundred. A thousand square feet to the gallon, and you can tint it ten percent toward your colour. And High Teck's other high build settles the question people ask here. In the sheet's own words: seal it before top coating. The high build doesn't replace the sealer — it's the step before it.

No spray gun? The same four, in cans No gun? All four are in cans, etch included. The two-K epoxy is a real two-part product: turn it over, press the red button in the cap, shake two minutes each side. Two passes is two point eight mils, and it sets in the can in about four days. The two-K primer filler is the high build: two to three passes, sandable in an hour. Six to eight inches off the panel, two minutes of shaking, on all of them.

One can, three film builds And here's the whole video in one aerosol. SprayMax's one-K self etch filler gives three film builds on one sheet. One coat is a weld-through primer. One to two is a primer. Two to three is a filler primer. Same can — you decide which product it is by how many passes you make.

Five mistakes Five mistakes. Body filler straight onto etch: not one etch sheet we read names a time for it. The epoxy sheet names one hour. Expecting etch to fill: six tenths of a mil, and the scratches are still under there. Missing a window: epoxy to seventy-two hours, sealer to twenty-four, surfacer to eight. Past those, you sand. Buying a fourth can when the one in your hand reduces into it. And skipping the sealer the high build's own sheet asks for.

Where to get it Everything here is at autofxmart.com: the etch primers, the two-K epoxies, the high builds and their activators, and the sealers. Most of these sheets say professional use only, and every product page links its safety data sheet: a respirator, gloves, eye protection and real ventilation, especially with anything two-K. Read it before you spray.

Primed. Now the colour. Once it's sealed, find your colour, mixed to your car's code, at autofxmart.com. And whatever your primer is, the number that matters is on its own sheet. Go and read it.