I would like to second PP's suggestion on the tubing. On my Frandee I had a bunch of square tube that was rusted through, generally on the bottom of horizontal runs. Also, on vertical runs where water could enter, collect at the bottom, then freeze and expand the tube, generally cracking it at the edges. One solution to this was to drill drain holes at the low point of the vertical run, however this then opens up the inside of the tube to potentially more moisture, whether in liquid form or humidity in the air. On the Frandee I replaced the worst of the tube, then cut out and patched the bad bottoms of the square tube in isolated spots. This worked, but not the best, mechanically is strong enough but hard to get the aesthetics to original back, maybe a better metal worker could get it to look original. I would not scab anything on the outside, my 542 had that and it looked like sh$t and was a point to collect debris, whether gunk or snow. When I encountered the same corrosion on the 542 bed rails I just completely cut them out back to good sound steel then replaced with new tube, was much easier and turned out way better (you can use the original tubing as a template for hole pattern drilling, etc. When I cut out the original rails, some were heavily corroded on the top, not the bottom, where the wood decking contacted the tube. The wood held the moisture and propagated the corrosion. I also had two horizontal rails that were packed full of "mud", to this date I do not know how that material got inside and so packed in. But I also had some rails that had absolutely no rust at all on the inside, looked like the material had just come out of the steel mill, these rails had been sealed up tight and had no holes for fasteners or anything else. Additionally, from what I have seen on my cats is, the aluminum panels where fastened directly to the steel frame during original fabrication, then the entire unit was painted. As you know galvanic corrosion develops between the aluminum and steel, especially when moisture is present (sounds like snow or humidity in the air). My Frandee had extreme lose of aluminum from this corrosion, it was "rotten", I had to fabricate completely new aluminum panels for the entire unit. To prevent/minimize future galvanic corrosion I painted the steel frame with an epoxy primer then with urethane paint before attaching the aluminum panels. There is still some aluminum to steel direct contact in areas, especially the aluminum drive rivets, but the corrosion should be considerably less. For the 542 bed I am going to seal the wood so it does not collect and retain as much moisture, as well as paint the frame before attaching the wood. It is a lot of additional work, and may not be worth the extra cost or time to do this isolation between the materials for you, but it is something to consider. If you want to see what I did for the 542 Cab Forward frame tube replacement, there are pictures of the work on the forum for it's restoration. I also believe I posted the tube size and thickness that was original and that I used. This probably won't help much with the Kitten since the 542 used what I recall was 1-3/4 square tube. Good luck, and happy welding. One last thought, I went over the entire frame with a small hammer and tapped on the steel to determine what tube may need replaced since as PP describes the corrosion can start from the inside and may not be evident from the outside.
542 Frame Restoration:
Then the bearings were drilled and tapped for the grease zerks, and a new bottom shield plate identical to the original one was attached.
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