Here's what happens when you use incompatible products:
I painted one of my trucks about five years ago. There was a section on the roof that had some very tenacious rust. I sanded it down quite a bit but didn't want to get into sandblasting on an assembled vehicle. I couldn't seem to get down to bare clean steel. I had used "Rust Treatment" by Permatex (from NAPA) on the rusty areas, which extended from the windshield back about a foot and all the way across the roof. I covered the Rust Treatment with a lacquer-based primer/surfacer (DuPont's 131S), the clean bare metal areas with epoxy primer (PPG's DPLF) and topcoated with DuPont's Centari Ultra. Everything seemed to be just fine. I can't normally see this area as it is over my head. Recently I needed to replace a couple of the overcab lights which were not working. To my surprise, the top photo is what I saw. After some grinding with 24grit, things turned worse as you can see in the lower photo. The problem only occurred in the area I used the combination Rust Treatment and lacquer- based surfacer. It seems to me that these products are incompatible with each other and caused the problem. The over-the-cab portion of the dump body shades much of the roof except in the problem area, so sunlight may have had something to do with it but I doubt it's the root cause. I have used that same Rust Treatment on my AC-B gas tank, but unlike the truck, that was used in combination with PPG's OMNI urethane surfacer, MP182. There has been no problem there in the four years that's been applied. Both vehicles are outside all the time. Going back and reviewing the application instructions for the Rust Treatment, I do not see anything that I didn't follow as far as I can remember. There is no warning about incompatible lacquer-based products being used over it. However, I have to conclude that they are, in fact, incompatible. Now that I have some actual rust throughs in the roof outer skin, I need to come up with some kind of fix. I don't want to get into cutting and patching by welding in pieces. I don't need anything that good. And I don't have the time for that anyway. I am thinking of a fiberglass overlay of the bad area - probably all the way across would be the simplest to do. The resulting difference in surface level is not important to me but I can feather it out somewhat. I can't see the area normally anyway. Any comments or suggestions on ways to proceed are welcome.  Rod
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