Of course it really needs to be clean of any rust, dirt, grease,etc. for a permanent repair, but I would rather use rustoleum in a can than a MCU because I've seen the damage it can cause even when using their procedures to the letter. Adhesion is not good to a clean metal surface, and any moisture between it and the metal will accelerate the rust that is there. So if you have clean metal next to rust, it will let loose of the clean metal and let moisture underneath causing more rust where there was none before. I understand a customer insisting on something, but telling him you will not be responsible may not be good enough when his frame is rotten beyond repair. He chose you to do the job, not his friends. You have to insist on doing what you know is right, because your name will be on it, not his friends. NEVER let customers choose the material you have to stand behind, because if it does fail, it will be your fault, because his friends told him it was the best thing going, and you must have done something wrong. Please don't take this reply the wrong way, I'm just trying to keep you from making mistakes I've learned from the hard way.