Even though it's a "beater bike", I'm guessing you really want to keep it around for a while. If that's the case, I believe the best protection for it would be to paint it.
Strip the frame down (take all the components off) and thoroughly wash it.
If you want to do the job yourself, sand the frame down to remove any rust spots. If you have too much rust and you think epoxy might be in order, you can do that - but I suggest being extremely cautious at that point. If the rust incursion is that bad, you may very well have structural damage that could cause a nasty crash - or worse.
Use "Frame Saver" and spray the inside surfaces of the frame down with a liberal coating while it has been disassembled. Clean the surfaces again, Use rubbing alcohol to remove any oily deposits.
Use a quality exterior paint.
Alternatively - almost any autobody shop that has a paint booth can paint the bike for your fairly cheaply. They might even have sand blasting facilities/capabilities to make cleaning the metal easier. If you don't care about the color - just have them paint it when they do a regular paint job - and it'll be even cheaper since they don't have to clean their paint guns for a tiny batch color job for the bike.
Smaller/independent shops will tend to be a lot more willing to work with you on a project like this. The big shops tend to not be too friendly to "side jobs" like this.
I've had steel frames completely stripped via sandblasting, and professionally painted with powder coating, including two layers of protective clear finish, then baked in an oven for 2 hours. The cost was $130.