Tops and sides. Don’t do the bottom. Buddy years ago did one of my ATV Trailers, it did good. It was free so it was done. All my other wood trailers, I buy the cheapest hydraulic fluid and get to rolling it, let it soak and roll several coats. Will last for years to come. Have a dozer trailer that my grandpa used in the 50/60/70s still in use, same boards.
I used 1/2 diesel and 1/2 used motor oil on my utility trailer/car hauler after I replaced several boards. It’s only been 6-8 months though. Several coats (3-4 on the new stuff) and that old wood drank it right in. Trailer looks better than it did when it was new. We’ll see how it holds up over time