Re: galvanized trailer rust
If the job is relatively small or in hard to reach laces where grinding is not possible, sand as much as possible and apply a good liquid rust transformer (not rust inhibtor). Rust transformer chemically transforms rust into a stable compound, turning it to a shiny black material that won't peel or chip. Enough needs to be applied to chemically react with the amount of rust to be transformed, it's easy to see after dry if completed. Rustoleum makes a good over-the-counter transformer. Paint over the tranformed area. Even if grinding to bare metal, I still use the transformer to make sure all rust has been removed, it acts as a primer.
