I need some advice from the experts.

I don't claim to be any kind of expert but I'll start things off with a couple of comments/opinions.

Your situation is not that uncommon. Yes, you can spot prime as you go - especially if you use a two-part epoxy primer. In my opinion it is the very best bare metal primer you can use. The stuff is impervious and once applied you can use the tractor and leave it out in the rain all summer if you want without any recurring rust problem where the epoxy was applied. The only issue that you would have with epoxy is that most of them have a definite recoat period during which they must be topcoated (w/o additional work being required). That period is usually a few hours up to seven days or so, depending on manufacturer. After that time has passed, without topcoat, most recommend a suff sanding (typically with a Scotch-Brite hand pad) and re-application of the epoxy prior to topcoating. That's to get the best adhesion possible with the topcoat. You would be in that category. I don't think it's a big problem but you should be aware of it beforehand.

Is there some particular reason why you wish to move from Martin Senour to PPG or DuPont? I use PPG a lot myself but also like DuPont. I have never used Martin Senour products but have no reason to think they are inferior in any way. I know they have several products lines of cost/quality just as PPG and DuPont do. And they also show an epoxy primer. I don't have an MS-to-PPG or MS-to-DuPont cross reference. However, at the MS website they provide a cross reference to their formula numbers from those of other manufacturers. I do have a PPG listing here for Oliver, although I don't show any red. That doesn't mean it doesn't exist...just that the list I have doesn't show it. There are several PPG numbers for Oliver Greens and Yellows. You could try plugging those into the cross at the MS website to get their equivalent number and compare it with what you already have. Working backwards in otherwords. Frank Stalfire is a DuPont rep that goes by here and he might be able to help you out with DuPont numbers, which you could also backcheck to MS numbers via their website. Do you know who is going to do the final painting and what he prefers to use? That might be an indicator as to what manufacturer's line of products would be best for you to consider.

Try not to use rattle cans. You won't be able to get the two-part epoxy in them anyway. Lets leave the water-in-air matter for another post in this thread. A lot can be said and I don't know what you have for equipment. I am sure there is a reasonable way to take care of the problem, particularly since you will be doing things piecemeal, a little at a time. Explain a little more just what you have for a compressor arrangement and we can all proceed from there. And if you are priming just small areas, there is a way to do it using the same epoxy w/o using a compressor at all.

Rod

Yes, NAPA is a Martin Senour jobber, at least in my area and I think generally also. I don't particularly care for the "cut rate" auto parts stores that have sprung up around here too. They seem to cater to the "homeowner" types rather than to "mechanic" types. Plus, much depends on the person behind the counter. My local NAPA has a great oldtimer mechanic that runs the place. It even has a couple of very knowledgeable gals behind the counter. These people know what they are selling, are extremely helpful to customers where a quick look-up in a catalog is not productive and generally cater to the more hard core mechanics among us. I have not found that with the cut-rate competitors.

The epoxy primer that I use now (PPG OMNI MP170) is extremely easy to use and should not pose a problem for anyone with minimal experience. The required catalyst does not contain isocyanates so a standard cartridge mask respirator is satisfactory. It is a two component mix by volume with a catalyst (1 part epoxy, 1/2 part catalyst) and once mixed has a pot life of about 8 hours. You do the mixing, not the place where you buy it. This means you don't mix more than you plan on using the same day. Anything left over after 8 hours is waste.

As you describe what you are planning, a spot gun would be the best choice, in my opinion. If I were going to paint a tractor in pieces, that's what I would use. I have painted most of the sheet metal on my AC-B with just a spot gun. I wouldn't use it for major broad areas such as a car hood, but it works just fine for smaller parts done individually. In fact, all the new orange paint you see here was done with my spot gun. Mine is an expensive DeVilbiss but for many uses, an inexpensive one should to the trick, especially for primer. I don't have a particular brand to recommend. My only experience is with DeVilbiss. The spot guns also use less air which is beneficial for marginal compressor capacities. Absolutely no need to fire up a regular sized gun for individual parts or relatively small areas.

For small areas, a Preval sprayer is also a possibility. They are inexpensive (less than $5 for just the powerhead - which is all you really need) and do not require a compressor at all. They are similar to a rattle can but can use any custom mixed paint (except fillers/surfacers). I would categorize the spray as a little "softer" than a typical rattle can. I have used one to spot prime with the MP170 so I know it works with that. You don't have to use the glass cup. I've mixed up small amounts using tablespoons, a 3 oz paper cup and just stuck the pick-up tube in it. Makes clean-up a snap. I wouldn't use it for larger areas but if you do a square foot or two at a time, it would work nicely.

Discuss the whole thing with your paint guy. I'd tend to let him do the final coat of epoxy (if you go that way) just prior to the topcoat and you only prep what you've done previously with a light sanding or scuffing to save some effort on his part.