by Squigie » Fri Nov 12, 2021 12:06 pm
If you're not worried about the steel conducting cold (sapping heat), I wouldn't worry about it. Seems fine.
You lose the benefit of a monocoque and frame strengthening each other, since the overall structure is now two walls and a roof attached to a frame. But it would probably be fine.
As for internal corrosion in the steel, that can be dealt with. There are many ways.
(You replied while I was typing, and mentioned capping. But I'll leave what I wrote. It was more of a reply to Pmullen, anyway.)
Fully sealing it is the simplest concept.
But there are others.
One that I have never seen used before, and honestly have never priced the tubing, is using A588 'weathering' steel for the frame. It oxidizes to a dark red/brown and then slows tremendously, due to copper content, even if fully exposed to the atmosphere. Very stable, long-lasting stuff. Commonly used for bridges, buildings, etc. It welds wonderfully, too. I'd like to see someone use it, but don't know if it is viable price-wise. I use it in corrosion-prone locations on vehicles, when fabricating brackets, mounts, etc.
One method that I have used in the past was arguably two, combined.
Wasn't a trailer. Rather, part of the structure for a porch. (But using the same 2x2x1/8" A500/A513 square tube so common for TDs.)
I sprayed inside the tubes with a water-displacing penetrating oil. (CRC Knock'er Loose)
I let them bake in the hot summer sun for a few hours, then sprayed again.
A few more hours of "bake" time really let that oil work into the pores.
One end was welded closed, or welded to another portion of the structure. If possible, the inside of that weld was sprayed/fogged again.
Then I purged the tubing with C25 gas (75% Argon / 25% CO2) and welded the other end cap or next piece of tubing in place.
About 18 months after completion, I had to drill two holes to add another element. The hiss was very satisfying.
Until I realized that it was a partial vacuum sucking cold, humid air in. [not so smug face] So, I drilled a little drain hole* at the bottom of those two tubes and we'll check again in a couple years. (They were on a 15 degree angle.)