boiled linseed oil is an inexpensive, effective bicycle frame rustproofing alternative to the products previously posted. its available in the paint section of almost any hardware/walmart. a quart costs ~4 bucks and will do many bike frames.
linseed oil stinks until it drys, and it takes several days for it to dry, so you need a garage or secure outdoors area for your frame to hang for a few days. so don't plan on doing anything with it for 3-4 days.
when rustproofing, use newspaper to plug the holes. then add lots of treatment. slosh it around everywhere to completely coat the inside of the tube. remove plugs, drain tubes. do one tube at a time, less likely to miss a tube and control the mess a little better. also have lots of newspaper on the floor, or better yet do it in the backyard.
linseed oil won't hurt your paint or powder coating. but wipe off the excess when its still liquid, because its very difficult to remove once dry.
linseed oil is also useful as a spoke lube / nipple immobilizer when building wheels. a little messy, but so is loctite / expensive dt goo / anything besides plain ole spokes and nipples, which work pretty good if you get the tension up.
leftover linseed can be used for painting...