corrosion: fix or replace?

[Chuck]

I spent some quality time yesterday with a grinder and a wire-wheel, scraping the rust of my frame, now that I have the floor removed.
Its a bit worse than I could see from underneath. The insulation that was sandwiched between the frame and floor did a number on the tops of the main frame rails; you can see it reflected in the underside of the removed plywood (the rot being much heavier/deeper, and in a straight line quite far in from the back of the trailer).

Anyway, I'm wondering a couple of things: can this be repaired by splicing in some angle-irons?

Also, the bottom edge of the exterior skin that was covered by the trim is fairly heavily corroded...quite brittle. There are a couple of cracks, and it can easily crumble. I hope this doesn't mean I have to replace the whole sheet.

The rear hold-down plate--can it be removed without cutting out the rear cross member? It might be best to just replace the aft 2 cross members, anyway...
I'm afraid that de-riveting the skin from the u-channel and hold down plate will cause it to disintegrate. Or if it doesn't, re-riveting it back might.

new pics here: Floor Project 2011 - Photo Gallery

[Wabbiteer]

Unable to come up with anything witty to say. I've seen the missing plies of plywood run seven or eight feet up the main frame rails, leaving 1/4" then 1/8" deep furrows missing due to the fiberglass water wicking. I've seen the lacey lower edges of outriggers but less so on my '73 spars.

That sure looks like a good candidate for adding a new spar right behind what remains if the tank etc. clearances allow doubling up.

As pictured there may be a strong argument for LEAVING IT ALONE, no more picking at it, scab in a 90° angle sheet metal stiffener on the bottom edge by tacking it in on the vertical faces near the main frame rails AFTER POR-15'ing the rusty spar one good coat to build up and strengthen what remains.

Once POR soaks into rust & oxides and bonds, the next coat(s) gluing new metal mated up almost* resembles being welded. * I reference the snapped C-channel that lifted with the shell on mine while being POR welded to the tie-down plate. Then the pop-rivets used to hang the belly pan will tie the rest of it together for a long time, just leave clearances for any plumbing etc. running through the lightening holes..

My back tie down area is different than that you've pictured, there is no stamped metal spar at the last cross brace area, it has a 1.25" x 0.75" heavy iron "U" crosspiece to intercept the tie plate. And yes the aluminum shell resembled busted graham crackers once I had it apart.

Replacing the 'entire sheet' is a little too deep for the nice metal above the oxidized strip there, maybe just rivet in a reinforced overlay for the lowest four or six inches. However, I am thinking of omitting the entire back utility hatch and frame versus replacing the tiny sheet strip beneath the hatch as AS provided.

Aircraft Spruce has anti-everything etch/brightener & passivation supplies for aluminum I think you'd ought to consider investing in. ALUMIPREP NO. 33 or METAL PREP NO. 79 then ALODINE 1001. Anything that has aging & weathering on it and all the other susceptible spots get treated...

[wahoonc]

I don't have that spar either, I was trying to figure out where you took that picture.

I have the same channel that Wabiteer has, so that must have been a design change along the way. I am planning on replacing the lowest piece of skin on mine, it is not in nearly as nice a condition as yours.

Aaron

[Chuck]

You guys have the bigger models, so there were tanks in that space, right?
So I guess the aft x-member was done differently. There was nothing in the frame cavities on mine but empty space, and a couple of drain lines.

Still, though, the hold-down "plate" must have been attached to something? I just don't see how that is going to slip out of there without cutting out the aft x-member. The hold down plate is pretty crusty looking...thinking that it should be attended to, either by replacement or at least de-rusting and painting. Also thinking that it would be easier to get replacement floor in there without the hold-down in place.
Since I do want to put a tank in that space, I'd rather not have to put the back sheet of floor from the inside, in 2 pieces, because that would require a splice underneath the seam and that would push the bottom of the tank down all that much lower.

[Wabbiteer]

Ah, yes... those words unspeakable "replacement floor"...

The piece will leap out easily when the floor is no more.

[wahoonc]

Hold down plate on mine is riveted through the channel in the vertical and was bolted down through the channel, floor and frame in the horizontal.

Aaron

[Chuck]

I can see how it might be able to wiggle out, as the vertical part is angled back a bit, but I don't see how it could go back in around a solid piece of plywood...unless the x-member is removed, and is the last thing to go back in.

[Aerowood]

I replaced the original with an aluminum version during my floor replacement. I replaced the rear skin. With the type of corrosion shown on the picture the only really good repair is to splice in a new section of lower skin, and to do that properly it would be less work to replace the whole skin. The inside is most likely worse then the outside. Do you have any pictures of the inside?

The frame cross-member is defiantly gone on the bottom flange. I would not make a determination on its repair until I saw the top of it. I have a few pictures of that area on my thread below at the very beginning of the thread.

I would not start a repair plan until the whole area is opened up in order to get a good overall assessment of the whole area, from all sides.

[Chuck]

Ok, I'll go ahead and drill out those rivets (gulp) and let the chips fall (literally) where they may.