A rusted utility trailer is by no means past its useful life. In fact, repainting a rusted utility trailer every few years will extend its life. The key to a successful paint job is to remove as much of the rust as possible before painting. You may also want to seal off the rusty area with a commercial rust sealer before applying the paint so that rust doesn’t return in that spot.
Brush away as much of the rust as you can using a wire brush. The more large flakes of rust you can remove with the brush, the faster and easier it will be to remove the remaining rust with the steel wool. Generally, it’s easier to brush than to scrub with the steel wool.