Hi,
Check with Moss and The Roadster Factory about those patch panels. They might have them or can get them.
Or, depending on exactly how far up the fender the rust has progressed, it might be pretty easy to just fabricate patch panels to weld into that area,. I would think so long as the rust is confined to the area below the first ridge and deep concave area of the fender (i.e. rust is only in the lower, convex area, where it's most common) this might be a relatively easy fix. Just use the rusty old parts as a guide to make up patch panels (or have a halfway decent body shop do the repair for you). That's 18 gauge mild steel and the curve is not compound, so seems to me it would be a pretty straight forward panel beating job. There are three mounting holes on the lower portion of the panel in case the rust has obliterated them.
Have you explored beyond the fenders? They rust at the bottom due to two main factors. One is when the vertical splash panel behind the wheel and/or its rubber seals fail. The other common cause is the scuttle vent drains that dump water behind the panel.
Sometimes it's a combination of these two causes. Either way, dirt and muck get trapped in there, then frequently dampened, creating perfect conditions for rust. Add to that, the hidden areas were seldom very well painted originally.
Unfortunately, the rockers under the fender are just as vulnerable and usually are damaged at the same time. Worst case, I've seen severe rust attack the inside walls of that area, which are the footwells of the car and even the hidden part of the a-post area. These are much bigger patch/repair problems than the rockers and fender.
I know splash panel replacement parts are available from The Roadster Factory, and they even go a step farther and powder coat the metal panel now. The rubber seals are widely available from all the various vendors. Just be sure to use some sort of seam sealer as a finishing touch, too. (TR4 service manual shows generous extra sealing in this area).
Probably the best solution for the scuttle vent drain is to lengthen it's hose a lot and drill a couple large (hidden) holes through the rocker panel to route the drain right out underneath the car.
There is actually one more small water drain, from the rear corner of the hood flange (this is on the inner fender, right near the hood's rubber corner bumper/adjuster). It also drains down into the same area and might be better closed off entirely to prevent any water intrusion.
25+ years ago when I replaced the rockers on my TR4, I did a lot of extra heavy rust-preventative painting behind the fenders, and it really paid off! However, when I recently pulled the fenders off again, a lot of dirt and such had found it's way in and built up between the rocker and fender again due to poor splash panel seals. Thankfully the extra paint (and fiberglass rockers!) prevented rust from getting started. During the current restoration, I am planning to make the changes to the scuttle vent drain, too.
Hope this helps!
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