First of all you have to determine if it has already been coated or treated with something. That will effect whatever you are applying.
For the top side: It depends on what you are going to put over it. Whatever you use (since I am assuming you are going to be living in it) you want something that doesn't outgas, or outgases very quickly and is non-toxic. My experience is with open trailers and flatbeds so I will leave recommendations to others.
For the underside. Since you can't tip it on end or upside down it's going to be messy. These work with bare wood, don't know how they would work if the wood has been otherwise treated. IMO the two best coatings for the wood underside of a trailer are:
- A mixture of1/2 boiled linseed oil and 1/2 turpentine. Applied in the hottest, driest weather you can and works best if the wood is completely dry. Apply, let dry, and keep reapplying until the wood will not absorb any more. Check each year to see if the wood will absorb and if so reapply. Usually have to reapply after a year and is good for ~ 10 years before it needs reapplying.
- Used motor oil (my favorite). Used as is or thinned. If thinned with turpentine it is a coating that doesn't get your hands dirty when dry. Apply as with linseed oil/turpentine. Some flatbeds that are 60+ years old still have solid wood floors (and this is with only the initial treatment, no reapplying).
An acquaintance used truck bedliner on the bottom of his contractor trailer. Still too soon to tell how this works, still going well after 5 years. Although he has had to reapply to the front 3 feet of the underside as this area gets blasted by what the tow vehicle throws up (which is a problem for any coating that isn't absorbed).
-- Spiff
