After a classic massage (wheel brush, sandpaper, hand brush), these days i get excellent results with rust dissolvers, some are in gel form, which make them ideal for these. Then its a matter of wiping, cleaning with wet cloth (clear water from a bucket with a spoon or two of kitchen soda). Once dry, I spray sync rich primer on the surface, and where the welding heat will not be intense, I spray a layer of 2k black (bit overkill).
After welding, these days I use a sprayable wax that gaves me so far excellent protection on few cars, the brand I use come as a can with a small tube with a nozzle so you can "inject" from the side of the rails.
I feel with you as I will very soon enjoy the same job on my rusty 71.