This is the first tiny house we have built, so it was enough of a challenge to build that the documenting side was let slide to a fair extent. Regardless, I will walk you thorough what we did, and how we did it to the best of my ability.
We used as many natural, unprocessed, and re-used materials as possible, which made it less expensive but much more time consuming than buying everything from the lumber yard.
Step 1: Preparing the frame
As we used an old trailer frame to build this on, we started by demolishing the trailer, separating out the aluminum to be recycled. The plywood floor was saved to be re-used as the sub-floor. Then we went at it with an angle grinder and drill with wire wheel and wire brushes for the hard to reach parts. As we removed the failing old paint and surface rust we looked for signs of penetrating rust which would need welding to maintain the integrity of the frame. Thankfully, we found none.
So we moved right on to priming with a rusty metal primer, and then a coat or two of quality metal paint.