I plan to treat the insides of the boxed frame rails on my car. This is a very inaccessible area. I can insert an undercoating wand and spray material, but there’s no way to subsequently clean any prep material, so acid based products are out of the question. The steel is solid with no externally visible signs of rust, and what’s visible through a few small drain/access holes in the chassis appears to be solid with only a surface coloring of rust – the car is not driven in the rain (and hasn’t been in rain or a wet environment for at least 30 years). I just want to take any necessary steps to protect it for future generations.