Re: Trailer square rail repair. How?
How badly rusted is it? If it is just surface rust, take a grinder, with a real knarly wire wheel, and scuff all the paint and loose rust, and repaint. If it is real pitted, you need to get that replaced, and in doing so, check the rest of the trailer, especially the inside of the tubing for major rust. Seeing as you don't really know the availability of steel materials, I would assume you aren't fluent with welders, and steel cutting tools/torches. You should take it to a welder, if it needs to be replaced. They should have the right size tubing/channel on hand, as 3" is a fairly common size, unless it has some goofy width to it.
