For something the size of a frame, an oxalic acid bath is more practical than using Evapo-Rust because of the substantial price difference. Just pick up a tub of oxalic acid crystals (sold as wood bleach) at your local hardware store, mix it with water in a big tub (a kiddie pool works great), and submerge the frame overnight. Remove, rinse well, dry, and if you so choose, treat with the rust preventative of your choice.