I discovered after letting water stand in my RIMS tube a couple of days that the plug threaded of the stainless steel elements is made of zinc coated steel and as this picture shows, the plug surface will turn into a rusty mess. But the solution is relatively simple, just drain and dry out your RIMS tube after every brew and rust is not an issue.
My only issue with this simple solution is it seems like a half-way solution to me. And after a lot of testing with paint, petroleum jelly, silicone caulk, silicone windshield glass sealer, silicone aquarium sealant and epoxy potting compound I believe I found the best solution. The stuff that works best is Max MCR epoxy resin circuit board potting compound, silicone aquarium sealant came in a close second because it's thin and flows well. The other silicones are too thick to flow a thin coating across the surface of the steel plug.
Here's how we apply the Max MCR if you want to do it yourself.We are going to start offering pre-sealed elements in the RIMS tube kits we sell on www.kegkits.com.