That's first class and "the right way" to do it. A bit of a time saver which may increase your rust protection and longevity of repair. For spots that can't be reached or places out of sight out of mind that you don't want to strip, I had good results on a 71' chevy with some rust out. Interior places you can't get to to strip such as inside rust ed through panels where you have holes in it after you grind the rust off the outside: Put a rust converter such as POR 15 in a pump type garden sprayer and spray it inside the panel to wet down the interior of the body panel this will "convert the rusted surfaces" to a black primable compound. After this I mixed a 50/50 mix of asphalt solvent based roof cement and mineral spirits. I used the garden sprayer again and this spray to heavily coat the interior of rusted out panels. This leaves a thick, durable, slightly soft black tar coating. I drilled several "drain Holes" in the under/back sides of panels to allow water to drain out freely. Also used this method for undercoating and rusty floorboards. Used putty over bondo over "kitty hair" filler. 10 years when I sold the vehicle nothing had bubbled out and all was still solid.