Regarding the inside:
I've been talking to welders and the take on airtightness is that it's a fine idea, just a lot of work. So I'm going to make some proof-of-concept pieces (like, take six plates and see if I can make an airtight box out of them), and practice on a spare frame I have, and go from there. If all fails, I'll just clean it and pump it full of somekinda oil, and just resign myself to having to redo that now and then.
What I really like about the idea is that it's completely reversible. Shooting internal frame sprays (as opposed to just oil)... is not.
Regarding the outside:
I'm glad to hear that epoxy primers can be rolled. I don't want to spray not because of cost (I have all the equipment, including spray guns), but because of local air pollution.
So here's my plan so far (for the outside):
* wash with degreaser,
* pressurewash with straight water
* pressurewash again to get any remaining sand and metal chunks and other crud, dry thoroughly
* roll two or three coats of 2-part epoxy primer, let them cure as long as necessary
* roll two or three coats of 2-part implement paint
* call it done, touch up in the future as needed
For specific product... name-brand Imron is too expensive. For implement paint I'm thinking Valspar. For the epoxy primer...dunno. Suggestions? I'm thinking I'll need a high-build one to fill in light pitting from existing surface rust.