Sealing the Underside

I am watching for the answer to this.. I have been wondering if I should get the trailer undercoated by an automotive place

First let me say that I have never lived in a wet area (or cold area) like many of you. Most of the trailer's that I have purchased, and have worked on, have come from the same climate that I live in. It's a mostly dry desert environment. Yet every "unrestored" trailer I have seen has had extensive wood floor and framing rot. Even the riveted skin type trailer's have the same problems, the floors rot, and the aluminum has extensive corrosion on the inside after 60 years from water/moisture/condensation.

Water runs down hill due to gravity. If you can stop the "external leaks", and "control the internal plumbing leaks", your about 90% on your way to trailer happiness. The "internal generated condensation" is the remaining 10% to the water/rot problem. The smaller the trailer, and the cooler the temperature, the bigger the condensation problem is. These trailers need to be ventilated. A slightly open roof vent does wonders. This is just my observation.

I have come to the belief that it's very important to seal the wheel (fender) wells with an undercoat, but if you look at any old car or trailer the cheap spray can aerosol undercoat just falls off in a short time. A professional sprayed on pick-up bed liner material might be worth the high cost for the fender well application. For the bottom of the plywood, I would just use the popular Henry's roof asphalt coating. I think even a "well prepared/sanded" and painted fender would work OK. I think it's import for the water that does get into the trailer, to have a way for it to get out. If the bottom of the trailer was sealed so well that the trailer would float, I believe it would cause even more problems. Again, in a different climate, I might have a different view point on this issue.

So.....Here's a possible plan for a retro fit, or a repair under the trailer. Trailer has a new or repaired plywood floor, under the plywood floor install sheet Styrofoam cut to fit between the cross members. Then cover the Styrofoam with a thin 1/4" sheet of plywood to secure the foam to the floor. Drywall screw through the 1/4" plywood, the Styrofoam, and into the trailer floor to secure. The 1/4" plywood would have the seal coating of the henry's.

For ours, we first started siliconing all the underneath seams. Next step was to clean and degrease the areas of the seams that were to be treated. We then used butyl tape that was 6 inches wide. Covered seams as we siliconed.I know a lot of people do not believe in using silicone, In this circumstance I believe it is ok.(it is on the undercarriage) Also made a patch for where the black water pipe came out with aluminum and self taping screws. (Kind used for metal roofs. Wont rust). Then covered with aluminum tape. Done...