RustSeal is best applied to bare metal where it can seep deep into the metal pores, interlock with it and cure watertight. If it is not possible to remove the old paint and it appears to be sticking well, you can apply RustSeal over it. You must first, however, wirebrush, etch or sand the old paint to create enough texture for RustSeal to bite into. Keep in mind that if that old paint should ever fail, RustSeal will peel off still attached to it.
If you choose to sandblast the trailer,
RustSeal can be applied directly to the metal without any cleaning or additional prep.
If sandblasting is not an option, remove all the loose flakey rust first, scuff the old paint and then follow the 3 step system for best end results.