I want to clean this up then varnish it. What would get the rust off without gouging the surface?

Fine grit shot blast or if you don’t want to pay someone a fine rotary wire brush on a drill.

Kind of depends on what you are doing with it afterwards.

Metal polish, like Solvol Autosol and large amounts of elbow grease 😕

Or a low speed drill/polisher with a polishing pad and/or brass wire brush.

To get you started. After giving that a go, a fine sandblast would be much quicker.

Try using a wire cup brush in an angle grinder, or a scotch brite pad also on an angle grinder, tis what I use to clean up my tools when they get surface rust on them.

Thanks. I’ll try a scotch brite pad in the angle grinder.