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Their PowerMesh product appears to be some sort of thin, pressed random mat; sold in small, single sheets, it doesn't seem very cost effective.

For years now, the supplier has suggested using a small piece of fiberglass, soaked with POR-15 to repair small holes. I recently tried it, but didn't like the results. The fibers didn't want to maintain contour with the surrounding metal - kept popping up. Part of that may have been due to my use of random mat, when the supplier recommended woven mat, but I think POR-15's cure time is just as much of a problem.

In contrast, I've often used the random mat with regular epoxy polyester resin - even backed by pieces of wire mesh to span a void - and had no problem, as the resin kicks off incomparatively short order. However, to achieve maximum strength, you need to laminate the steel with fiberglass mat from both sides; and having several holes through the steel to allow both layers of fiberglass to bond to each other (much better than they bond to steel) - helps lock it all together. I find fiberglass and resin much easier to work with in this type of repair.

Yet, I suspect that POR-15 bonds better (with proper surface preparation) to rusted metal than the resin does. In fact, in constructing the Avanti body, Studebaker attached fiberglass bonding strips to metal parts, with pop rivets, and then bonded the strip to the fiberglass part. Also, when cured POR-15 remains somewhat flexible. If the metal isn't full of holes and doesn't shatter from a hammer blow, a couple of coats of POR-15 may be all it needs.

However, why not just cut out the weak spot and replace with good metal? If welding is not an option, pop rivets may be another option - depending on your preferences/tolerance. A body man once told me that, properly sized/placed, pop rivets are as strong as spot welds.

Out of necessity, I started doing fiberglass repair overlays back in the 60's. I grew up in the winter salt zone. There's 2 issues, you have to remove the rust area back to solid metal and have a rough surface ~60-80 grit for bonding. Epoxy resin is better but polyester works well, easier to sand. I wouldn't recommend sandwiching rust, unless you don't want it to last very long. Rust expands, pops the patch. Must be sealed from oxygen and moisture. Rust will continue once started. You can patch with steel and rivets just need to hammer and lightly grind so there's no bulge. Any attempt to just cover with fiberglass on anything but small holes will lead to thick filler requirement. Those are the easiest to cut out and weld anyway, so why? In the bondo queen era, they used to try to use screen and bondo to patch holes and it was an abortion. That's why I invested in welding equipment... Best patch life is around 5+ years depending on climate. I used to use galvanized sheet metal(before one sided) so the backside was protected. I recently repaired my p/u door, exposed to salt and it's been holding nicely in this climate. If you use rust preventatives, pre-layup, you may encounter bonding issues. Some coatings will adhere to primers, some won't, or the coating solvent may lift the primer. Lead has fallen out of fashion but quite effective and period correct.

Oops, I meant polyester - not epoxy - resin in my previous post. Now noted and corrected.

That's some excellent practical advice and experience that you provided, there, Jim.

Yet, I did not mean to imply that you could simply throw fiberglass and resing directly over the problem area without some surface preparation, but you can't do that with POR-15 either - else, it won't last for long, either.