My horse trailer was originally painted in urethane enamel. It has multiple rust spots that I'm in the process of removing. I want to repaint the interior - for some reason, the interior has all the rust, not the outside...

I have already removed most of the rust, with the exception of the crevices. Suggestions on getting the crevices clean? The ribs on the ceiling have rust that is hard to get at.

The majority of the walls have a decent layer of paint without rust, i was hoping to sand and paint over that. I was planning on using rustoleum enamel. If i sand with 400 grit, will the rustoleum adhere okay? Assuming that is the right grit sandpaper?

And what primer would you recommend? This is my first time doing this myself so please consider me a newbie.

If you want it to last I suggest you have the trailer media blasted to remove all the old paint and rust.

Then shoot two coats of a quality epoxy primer. After that a Single Stage paint would be my choice for a horse trailer.

The problem with paint over existing paint is that your new paint is adhered to the old paint which is already failing.