DIY: Frame Touchup
The time to touch up nicks and scratches on the frame is after the old straps or slings have been removed and before the new ones go on. The method to use depends on the type of metal to be painted. For aluminum, rough up the surface slightly with 100-grit sandpaper, then smooth with 220-grit paper. There's no need to prime — just apply an exterior-grade acrylic enamel paint. Use a tiny touch-up brush on nicks, a spray can on bigger scratches.
For wrought iron on tubular steel, use a wire brush and sandpaper to sand any rust spots or chipped paint down to bare metal. Next, brush or spray on a metal primer, then apply an exterior enamel topcoat. (A coat of rust converter such as Rust Reformer stabilizes rust without sanding, but a rough, crusty look will remain.)
When a painted frame is covered with scratches or caked with rust, it's probably time for a professional overhaul.