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Rust pits are difficult. I sandblast them and I think that is the best way. However, lacking that as an option, I guess I would try the Rust Mort or a similar product. The Rust Mort doesn't have to get a wash rinse like the PPG and DuPont products (if you follow their written instructions from the tech sheets) and is a one-step application (no follow-on second product). I am not a fan of etch primers but that would be another possibility without blasting. I don't think that a product like "Extend", which I like, would be appropriate here because it really should have some significant rust to apply to and not minute bottoms of scattered pits. I have no experience with the Mar-Hyde product. After primer, yes by all means, go over the pitted areas with a "bondo type" polyester filler. 3M (and others) make a thinner filler that comes in a squeeze tube. You still need to use the activator though. That stuff is almost flowable and if the pits are significant, such a product does a good job. You may need multiple applications. Lacquer spot putty, frowned on in today's high speed competitive world, can also be used successfully if done properly. Sand and prime again. At this point a surfacer is usually used prior to the topcoat. That is a high build, easily sandable "primer". You have to get the surface glass smooth prior to your color as the smallest imperfection will be magnified and show up after the color is applied. A 400 grit sand would be about the coarsest prior to the color. Some would say 600. Sounds like 3M is quoting from some lab tests. I am aware of some basic lab testing concerning iso removal from cartridges PROVIDED they have the prefilters in place. Of course that also presumes a perfect fit with no leaks, including the exhale valve. In the real world, I find that difficult to obtain on a dependable basis and with facial hair it is simply not possible at all. I would be interested to know if 3M has any independent approvals from NIOSH on it. I almost doubt it. It would be better than nothing at all. There really is no quick cheap fix here and no positive substitute for supplied air. A personal choice however if you are not in the employ of someone else.
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