Totally get what you're saying about rust having a mind of its own...been there myself more times than I'd like to admit. One thing that helped me was going a bit overboard on prep—wire brushing, sanding down to bare metal, and then wiping down with mineral spirits before applying primer. Even then, it's never 100% foolproof. Rust seems determined to win eventually, but at least we can slow it down a good bit. Hang in there, you're definitely not alone on this one.
I've found that the primer you choose makes a big difference too. Tried a zinc-based primer last summer on a metal shed roof project, and it really seemed to hold up better than the usual stuff I'd been using. Rust still creeps back eventually—like you said—but it's way slower. Curious if you've experimented with different primers or coatings beyond just prep? Seems like the products vary a lot in performance, at least from what I've seen.
"Tried a zinc-based primer last summer on a metal shed roof project, and it really seemed to hold up better than the usual stuff I'd been using."
Yeah, zinc primers can be pretty impressive. I've also had decent luck with epoxy-based primers—especially on roofs that get hammered by weather. Did a test run on an old storage building about three years ago, and honestly, rust hasn't dared show its face yet...knock on wood. Prep matters, sure, but investing in a quality primer is definitely worth the extra bucks in my experience.
Zinc primers are solid for sure, but epoxy primers aren't always the best bet, especially if you're dealing with older metal roofs that flex or expand a lot. I tried epoxy primer on an older barn roof once—looked great at first, but after a couple harsh winters, some cracks started appearing. Honestly, for roofs that move around in temperature swings, I've found flexible urethane-based primers hold up better in the long run. Just something else to consider...
"Honestly, for roofs that move around in temperature swings, I've found flexible urethane-based primers hold up better in the long run."
That's an interesting point—I hadn't considered urethane-based primers before. Do you think they'd also perform well on galvanized metal surfaces, or would adhesion be an issue there?
