A good finish needs to provide moisture protection, be durable and for exterior surfaces hold up to UV exposure.
A question I often hear about wood based camping trailers is, won’t they rot? Rot starts to form when bare wood is constantly subjected to moisture. With proper moisture protection, in the form of a good finish, this is not an issue.
When it comes to finishes, there is a spectrum of options depending on your appearance preferences, the amount of time you want to spend and budget. On the entry end of the spectrum there are acrylic latex paints, better known as house paint, that provide adequate protection. I was initially surprised to find them regularly being used as an economical finish for various small plywood boats. On the high-end spectrum are the glass smooth, marine paints and clear finishes that take a substantial amount of prep time and some practice to apply correctly.
There are two finishes that I use that fall in the middle of the spectrum. The first is a roll-on, textured polyurethane finish call Durabak. It is commonly used as a pickup bed lining and non-slip marine / floor coating. It’s very durable, flexible, UV stable, waterproof and available in a spectrum of basic colors. One nice thing about a textured finishes is how forgiving of surface imperfections they are.
The second is a clear finish to expose the nature beauty of the wood. I’ve been transitioning from using high-end marine spar varnishes such as Epifanes, to the new waterborne Alkyd Varnishes. I’m currently getting good results with Target Coatings EmTech 2000. It’s brush friendly, has fast recoat times, is UV stable and good for both interior and exterior coating.
With both finish types, I normally start by sealing the exterior on all sides with a penetrating epoxy. It acts as a grain sealer and provides an extra measure of moisture protection. I use Smith Company’s Multiwood Prime, also know as CPES. It’s watery and has additives that help it to be drawn into the pores of the wood. Give extra attention to end grain areas, they will absorb more than other areas. Be sure it is fully cured per directions before top coating.
Another practice of mine is to use the Durabak finish on the front of trailers that I’m doing a clear finish on. The recycled rubber granules that give it its texture, makes it very chip resistant.
To really extend the life of your finish, ideally you want to store your trailer indoors when not in use. If it’s necessary to store it outdoors, use a cover that doesn’t allow water to pool on it, blocks UV exposure and is loose enough to permit air flow.