I have a 17 year old Transcraft TL-2000 80,000 lb flatbed trailer, which seems to have comparably little rust for its age.

Prior to me buying it, it was owned by a construction company, and probably was not used much during winters.

The bottom line is that it does have rust, but it is superficial, no rust pancakes falling off, no through holes etc.

My question is how to keep it from rusting. Maybe I should spray the frame with some heavy oil, like hydraulic oil, every couple of months?

thanks

You could get a steel brush and get all the rust off and then use a good primer and then a good paint. If you stay on top of it, you can keep the rust down. Another way to reduce rust is to wash the trailer as soon as possible after snow.

You can use petroleum jelly to remove it after cleaning the terminals and also keep environment dry as moisture increases corrosion.

I bought an old Ford Pickup that was in similar good shape for it's age except for some surface rust in the bed... put a wire wheel on a drill and brushed off all the rust I could hit with it. I then bought some "Rust Converter" paint in a spray can... Sprayed the Rust Converter paint on all the old bad spots that had rusty color. Then applied a Rust Enamel paint.. worked great.
You can use a wire wheel on a drill like I did or a grinder if you've got some tougher spots that have bubbled up.