While pulling off the exhaust manifold today I noticed a few (smaller than a quarter dollar coin) patches of rust on the subframe of my 745. None of the parches seemed soft enought to poke a screwdirver through, but the rest of the body looks pretty nice and I'd love to forstall the eventual heat-death of the universe...

So, what should I do about the rust? On old Saabs I used to just spray rubberized undercoating over the whole wheel wells once a year. Is this enough or should I polish with a grinder, prime and then coat with 3M spray-on rubber?

I think the current posts are great, and I agree with them wholeheartedly, if the patches are already bad. Covering any rust with just the undercoating is not good. If the rust is not converted or removed, it will continue to expand under the undercoating. If the rust is just light surface rust, 1/32 or less penetration, consider a couple of products: First, for light stuff, there is a product by Permatex called 79DA. Clean off the loose stuff, but don't remove everything, or you'll have nothing to convert. And of course, if you removed everything, you don't need a rust converter. I bought this stuff for a barbecue a couple of years ago. Basically you remove loose scales, spray it on and let it dry. Then you can prime and paint. I sprayed the spot on the barbecue and then forgot to finish. BBQ was pushed into a corner all winter. When I pulled it out in Spring the spots were still fine. This stuff does need to be kept out of UV. A heavier duty solution is called Rust Mort. Find it at body shops only. Prep, brush on, sand, fill, whatever prep you like, then prep and paint. Also UV sensitive. Next step up is the "P" solution as noted.

DS

Scrape it,wire wheel it,put on a respirator and clean it with
lacquer thinner to remove the undercoating/oils.
POR metal prep {phosphoric acid}
2 X POR-15
Re under coat.
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White 86-245 DL, M 46, IPD bars & Wagon Overloads,Commando Bumpers,SS Belly Pan & Air Pickup,Straight-Shot EMT Chassis & Tower Braces,Scorpius Alloys,2 Belt No AC Conversion,Black POR-15 No Glare Front End

Ditto what Redwood said.

POR-15 is remarkable stuff. You can get what they call a "starter kit" which includes a small container of POR-15 "paint", and a small container of POR-15 "Metal Ready". The kit is $15 at many online retailers. The kit is perfect for the average shade tree mechanic who is doing occasional touch up work on a couple of cars.

Good luck.

jp