My first chassis (a 2012) I did the anti corrosion wax treatment inside the tubes, never quite got round to doing it on the second chassis but did have a look inside several of the tubes on the third (and current!) chassis with a borescope. The signs of brown-ness that I saw did rather encourage me to repeat the treatment with another can of Dynax S50. I'm sure your garage will be able to sort out the problem areas on the outside and get whatever their wax preference is into the tubes to at least slow the internal corrosion process. Stopping carbon steel from corroding is a bit harder than slowing it down unfortunately.
If it is any consolation one of my upcoming jobs is sorting out the stone chips on the forward sides of the wishbones and the cross tube under the engine.
