best rust converter?

depending on the amount of rust might be a good idea to have it sand blasted then the rusted areas treated with the acid. if your planning on replacing the qtrs. cut them off and get the inner structure real clean. same goes for the inside of the frame rails cut out the bad and blast then seal. as far as welders go i'd get one with a bottle not a flux core wire feed you'll have a lot better results and wont come back to haunt you. wire brushing the underneath will only smear the undercoating around if it has it. if it does spend some time with a propane torch and a scrapper. I personally before attacking the car with cut off wheels and grinders would remove the undercoating 1st.atleast around the areas you plan on keeping. good results are mostly achieved by time and proper prepping