I had the same problem. The insulation above the belly pan has just enough weight to aggravate the problem. I re-installed the belly pan using large outside diameter washers and pop rivets. I also supported the center of the sheet by installing an "L" shaped aluminum extrusion from frame rail to frame rail.
Best thing you can do is pull out all the stinky pink stuff. Most frame rot is caused by that stuff staying wet and laying against the frame. Also the wood can dry better without that stuff in there. Unless you live in Maine or Canada, you won't miss it.
I had the same problem on my '89 Excella. I made small plates instead of using washers. I bought some thin aluminum strap from HD. It was 1" or 1.5" wide. I just used a hacksaw and cut the strap into squares. Drill the appropriate size rivet hole in the center of each piece and rivet in place.
2005 39' Land Yacht 390 XL 396
Common Sense
, Texas
Join Date: Sep 2007
Posts: 5,319
Someone asked previously in the thread if there was some other material than aluminum that could be used underbelly, and last weekend I was helping my brother with his SOB fifth wheel trailer, and noticed it used black plastic sign material.
Not recommending this mind you, just reporting another material that is used for the purpose.
Glad I saw this, i'll be putting my new belly pan back on soon. I bought a bunch of 3/16" large flange rivets from VTS. I just looked in the McMAster-Carr washer section and found some zinc plated steel blind river washers for 3/16" rivets. Think i'll get some of those, 250 of them for $9.40. Seems like a inexpensive safe bet. They have round ones and square ones, probably just stick with the round ones, even though the square ones would have a little more surface area. Then i'll shoot them with a little cold galvanizing paint.
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