I've taken several scope bases off of several different rifles and have found rust underneath them. The rust ranged from light to fairly heavy pitting. The price of the rifle doesn't have a lot to do with it either. Some of the worst I've found was on a KDF (Kleingunther) and on several different Brownings. But down in Alabama where I did my work, it was very common to hunt in the rain.
Because of this, about 25 years ago, I started painting the underside of the scope base, and the top of the receiver where the base would sit, with clear fingernail polish or clear model airplane paint. Then while the polish or paint was still wet, I'd set the base in place and tighten the mounting screws, coating the threads with Loctite as usual.
A few times, I've been caught out of Loctite and I just coated the screws with the polish or paint that I put on the base and receiver. That held the screws tight in every instance, however it's not as secure a grip on the threads as the Loctite is. Once the base is tightened down, I go around the perimeter with a cotton ball or swab dampened with acetone to remove any excess that has squeezed out from under the base.
I always degrease the base and receiver top with acetone or Brake-Clean or carb cleaner or something like that before painting it. This insures a good bond of the polish or paint to the metal.
A couple of times, I've used epoxy to seal the base to the receiver. But I had to remove the base from one of the rifles a year or two later, and it was very difficult breaking it loose from the receiver, and then cleanup of the dried epoxy was difficult and time consuming. Because of that, I stick with the clear fingernail polish or clear model airplane paint because, even when totally dry, it can be easily removed with acetone or fingernail polish remover (which contains acetone).
I've taken bases off of rifles that I have mounted and sealed this way, that have been hunted in the rain every year for over 15 years, and never found a speck of rust under one of them.