This is the year I've decided to fix the bottom edge of my 6 wide sport platform. I've had this trailer since 2009 and it's a 2006 model. It's got about 14,000-15,000 miles on it. I've done some mods and maintenance in the past. I've had one side (The road side, left) where the aluminum U channel covers the bottom edge of the side wall in front of and behind the wheel. When I took the channel off, which was falling off, I found that for about 2-3" up the plywood had rotted. The inside and outside walls had held things together but it needed fixing. I cleaned out the rot with a chisel. It came out pretty easily and cut a 2" wide piece of cedar I had lying around to slip into the place where the plywood was. I had to notch the ends as I ran into some bolts up there.
Next I had obtained a 1 1/2" aluminum angle bar 1/8" thick. This stuff is way to expensive. I cut a piece that would go from the front corner of the trailer, under the fender and back almost to the axel. Next I applied a very thick strip of white silicon II exterior caulk to the inside of both walls of the aluminum angle. I clamped it in place and drilled through the aluminum, sidewall, and both walls of the frame. I inserted 4" stainless steel carriage bolts with washer, lock washer, and nut and tightened it.
So far just the front of one side is done. I've still got the other side and the area behind each wheel. Another full day for sure. Then I'll go around and caulk any gaps or cracks to keep the water out.
The old girl is getting old and the wear is surely beginning to show. But you know, I bought her used, didn't pay that that much for her, She's given us many good times over the thousands of miles. When it finally needs to be sent off to someone who wants a very well worn camper it won't owe me anything.
I've included pictures of the dry rot, The channel taken off and cleaned out, the angle and cedar I cut to do the repair, and the new angle bolted on and sealed. It ain't perfect but with a little maintenance I'm betting it takes care of things for awhile.