So I am using this rust killer that says to wire the loose rust off and apply 3 thin coats of this stuff. However, it says not to use it on bare metal . What happens if it does? I don't see any way to use it if it can't touch bare metal.

It is a product called "Rust-Mort" from SEM

It probably means not to use it on fressh clean bare metal as a rust preventative. Only use it on areas with rust only, cuz it is supposed to react with the rust to turn it into zinc usually. Using it on bare metal might have a reaction with whatever you paint it with.... thats my guess without seeing the product

I used Rust-Mort, it's good stuff. If you get it on bare metal, it's not a problem at all, as long as you use a sort of conditioner afterwards.

John I have never used it but have read alot about the different products. The product will dry to a chalky coating when you get it on bare clean metal. wipe that off with soap and water once dry and you should be good to go. Hopefully the professionals will chime in.

Jim

Works well. Eats at the rust faster than por metal ready. I used it on my 75 battery tray. But I completely sanded it off before applying the por15 products for a final cleaner/sealant. It leaves a film if you let it dry on, then you'll have to reapply to get that film off. Not a body guy, but one reckomended it to me.

It turns rust to Iron Ferrite, IIRC. Once it's done its work....like the man said, soap & water & dry......your wife's hair drier will do the trick.
I use R-M etch primer (rattle can) over the that, then a good top coat .....not Rustoleum.