Metal Prep

http://www.fboerger.com/restoration%20tips.htm

Bet you'll be glad when you can change your first name :o). You are correct, the PPG phosphoric acid conditioning products (DX579 and DX520) are not really appropriate in your situation (they are not recommended for cast iron either). Rust Mort, also a phosphoric acid based product, is not appropriate either - at least in my opinion. Those products are intended to be topcoated with something right away. In your case, where you have the metal nice and clean, I would use epoxy primer - a single coat. You don't even need to dirty up a spray gun. Since you are talking reasonably small areas, just mix up a small batch and use a Preval sprayer out of a paper cup. I have even used tablespoons as mixing measures and a 2 ounce bathroom paper cup for this. No clean up to speak of either. That will provide excellent protection until you get ready to do more. You don't even have to finish a part - just prime a done area when you want to quit. Don't worry about the time window with the epoxy. When you are ready to final prime and paint, just scuff "sand" with a coarse Scotchbrite pad, reshoot the epoxy with your regular spray gun and proceed with either a surfacer, if needed, or your topcoat.

My current favorite epoxy is PPG OMNI MP170 because of cost. In the past, I have used PPG DP40, the older, leaded, higher-end ($$$) product on clean sheet metal without a topcoat that was always left outside in the rain for ten years without a spec of rust forming. I haven't that kind of experience with the MP170 but suspect it would also be good under such conditions.

Rod

On the subject of masks, I happened to call 3M technical support today and they told me I could use their model 07178 mask to guard against Iso's. They said the only issue is that Iso's are odorless and that it's difficult, without a testing procedure in place, to know when to swap cartridges. However, they said that the solvents will penetrate the mask before the Iso's. So, "in practice" I would be covered if I just swapped cartridges at the first moment I smell any solvent through the mask. Just to be safe, I think I will just skip the hardener anyway.

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.

It has a fairly "soft" spray - at least that is how I would characterize it. Somewhat softer than a typical rattle can. You would not want to use it for large panels nor try and do a whole tractor with it. It is no substitute for a real spray gun. It does have it's place though and is very handy at times.