Has anyone used POR15 as a first coat on bare metal in high rust-prone areas? I've been using it in rain gutters, under the window trim, and cowl area - letting it flash, then hitting it with etching primer. Before I get to the point of putting the final coat down next month, I thought I'd check to see if anyone has had problems doing this..
Thanks in advance.
'76 BMW 2002
Putting etch primer over Por 15 defeats the purpose of the etch primer. You may as well have/should have used epoxy over it. Etch primer has an acid additive that kills surface rust in the bare metal and provides a chemical bond to the metal as well as having some rust prevention benefits. If it's not in contact with the metal it's doing none of those things.
I, for one, am not a fan of using rust paints in the car finishing process. Most of these are rust encapsulaters, meaning they attempt to seal in the existing rust from the oxygen and moisture required for them to grow. In fact some require that there be rust present for them to adhere to. This is fine for outdoor furniture or your lawn tractor, but if you're refinishing/restoring a car I say get rid of the rust.
My process is to sandblast the infected area then apply a phosphoric acid based rust killer like Picklex 20 or Rust Mort. This kills/removes any microscopic rust left behind in the bottom of pits or in seams. I then apply my epoxy primer and continue on the finishing process. This has served me well and on my kids daily driver /testbed Cavalier the rust hasn't shown the slightest sign of return in over 2 years of driving in our sub zero temps and salted streets.
As to prevention in these rust prone areas, I would take an analytical approach and ask myself why these areas are prone to rusting. What causes it to start there in the first place? For example, the rust common in 60's el caminos in the lower channel of the back window. Looking closely at it, it appears that the rust starts on the outside of the vehicle, under the chrome moulding. Dirt, crud and debris run down the glass and collect under the moulding. Now the water running off the glass will keep that mess nice and wet, a perfect medium to work away on the paint and compromise it's protection of the metal below it. How to prevent the recurrence? Keep it clean. Different areas have different root causes, but examine yours and determine what needs to be done. Whatever happened to the spots in the past compromised a quality, factory controlled finished. First job is to determine if the rust started on the inside or the outside of the panel. If it's started on the inside or has made it through to the inside from the outside, even with just the smallest pinhole, the inside needs to be blasted and treated as well or you're just going to have it coming through again.
Just my 2 cents.