What is the best way to clean a seam on a rusted fender?

I am working on mig welding some cracks on a rear fender of a 1950 Ford Truck. There is a pretty thick layer of rust and I am having trouble getting welds to start and they like to pop and splatter allot. I am using a 115 volt mig welder setup for gas and using a 75% Argon 25%Co2 mix at a flow rate of 3 LPM. The wire electrode is .023. I have tried emery cloth and a wire wheel they get the loose rust off but are not getting back down to the clean steel. Would Muratic acid work for doing this? I have thought about getting the fender chemical stripped in a bath but hate to do that until I am ready to prime and put a coat of paint on to prevent flash rust.