i have an old cromoly frame, and there are some little rusty parts on the tubing but a lot of rust on the outside and in of the bottom bracket shell. i can thread BB cups in there but should i be worried about the rustiness weakening it? what should i do to it to "restore" the rusty parts before building it up?
Kirk Frameworks JKS-C, Serotta Nova, Gazelle AB-Frame, Fuji Team Issue, Schwinn Crosscut, All-City Space Horse
There are a number of different frame prep products to keep the interior of frames from rusting. A quick forum search should help you find the names. I'd probably go with a bit of fine grain steel wool to take down the loose rust on the threads and make sure to grease the threads before putting the BB in to prevent additional rust from forming and causing problems down the road when you want to get it back out. On the outside of the frame, I've had luck with a number of different products to remove rust ranging from WD-30 Alumnium Mag wheel polish, chrome polish, Naval Jelly (a very very strong rust removal product), and wet sanding. What ever course you go to remove the rust, be sure to treat the tubing with wax on the outside and a rust inhibitor on the inside to prevent more from forming.
The only way to stop rust completely is to remove it to bright metal and prime and paint. Sometimes you can get away with a patch. My Bridgestone had a former hard life offroad in Wisconsin. Muddy tires had rubbed the inside of the fork and chainstays bare and they had rusted. I wire wheeled the rust off and painted the areas satin black. Nobody notices the repair because it looks like a shadow.