Excellent replies, all, thanks! Yes, this is a 54K original mile car, and there is very little surface rust and about 1/2 the original undercoating remains. I have been getting the large deposits of dried dirt out of crevasses, and using a soft brass brush to get the loose dirt off and keep the undercoating on. I was thinking PB blaster or WD-40, but I don't want to loosen and remove the undercoating. I may be a little more daring inside frame rails where there is visible minor surface rust and perhaps use a product, but I don't want anything to "bleed thru" and be visible.
krelboyne: I have a couple of parts cars (
) sitting around that I can experiment on--great idea!
mgmradio: I'll work on the just getting the dirt off/out first, then I think I'll be getting in contact to discuss. I haven't had a car as unmolested as this to date, and I don't want to negatively affect its history and patina.
Thanks again!