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.
Assemble the pre-drilled electrical box, heating element, element gasket and 1" stainless X 1-1/4" stainless bushing.
Tighten the assembly - we use a vise and a water tank element socket for this step. If you don't have a vise you will likely need a friend to hold the other end. Max MCR is very thin and will leak past the threads if the assembly is not very tight, but this is why it works so well. The potting compound flows well, covering up the entire surface in plastic once it's cured.
Place the assembly element up on a flat surface - we flip it over in the same vise.
Pour and mix the epoxy resin per the manufacturer's instructions.
Pour a thin coat of the resin into the assembly - you should see it spread out and completely cover the surface of the steel plug.
But do-not fill the part up, any more than a thin layer will cause the compound to heat-up and burn when the water heater element is on.