This patch panel is just like the original and left like this the backer plate will trap water and rust out just like the original did. So, should I clamp it tight and weld it up? But water is likely to get in between the layers through the bolt holes. How do you protect the surfaces between the layers?

 

Spray with and acid etch primer (Duplicolor has it in aresol), let dry and then apply by "misting or fogging" Motor Craft PM-24-A or ValuGard VG-101A rust inhibitor. (ValuGard makes the MotorCraft product).
It does not require "heavy application", just a light coat. It seeks out any pin holes, bare metal and if water is present, runs it away and coats the area where it was.
This is also the product that MoPar provides, but they don't have it in an aerosol like Ford or ValuGard.
On the ValuGard.net website, under Engineers section are Shop/Body Shop repair processes for panel replacement, rust, etc for Ford Motor Co.
The ValuGard can is a larger amount and you can order just one can, with Ford, go to a dealer, but they may want you to buy a case.
Both companies also provide the only "undercoating" that is approved by several vehicle manufacturers and it can be bought in aerosol as well.
Undercoating "is not a rust preventative" in reality, it is an outer coating. People are very confused on the use of "undercoating".
You must use the rust preventative/inhibitor (some times called rust proofing), after paint work is done to stop the corrosion for years and years.
Unlike so many "rust preventatives" marketed, this one exceeds ASTM-117B standards and the new SAE-J2334 standard.
Which is why Ford, Chrysler, Hyundai, Mazda, etc specify the use of it for all metal repairs. (some TSB's from them are also on the site.
Just a heads up on how to solve you problem once and for all.

 

Try a cavity wax. Available at body shop supply stores.

 

Thanks for the info. Is the VG-101A intended for application in the crack between the patch panel and the backer plate? I was planning to sand blast and epoxy primer and then undercoat the inside of the fender. Should the VG-101A be applied before or after completing the primer and undercoating? I assume it is thin and will wick into the surface between the 2 layers of sheet metal? I'm thinking the patch panel was made by spot welding the inner backer plate to the outer piece and both sides of these 2 pieces were painted before they were spot welded together. Is the VG-101A probably compatible with the black paint that is on the patch panel.

 

Also, any problem with spraying this into the seam between body panels that are bolted together?

 

Based upon your post, I have to assume that you never went to the link I provided and read the information.
The product is "not undercoating", which does not stop or prevent corrosion for long term.
It is applied to any metal surface, but is best to be applied to a surface that has some acid etch/anti-corrosive primers applied first.
It will "creep" seek out bare metal and coat it with the rust preventative/inhibitor, remove any moisture that is there, and protect the metal for years-IE, ASTM-117B, SAE-J2334/
From you pictures, I suggest that you remove all the rusted surfaces surrounding the affected areas to insure that you have years of protection and no more rusting/corrosion in that area.
Undercoating is NOT GOING TO STOP FOR LONG CORROSION, it is there for other reasons.
You may apply an undercoating that meets ASTM and SAE standards over the "rust inhibitor/preventative" after it(rust inhibitor/preventative is applied.
Please take the time to read the "manufacturer provided" information on the provided link.
valugard.net, under the Engineering link.

 

How about, around 10yrs ago, my kid went through my fathers truck being a
1950 F6 my father bought new. Body off the whole nine yards new front complete fenders, grille surrounds, running boards all new my father bought
from Ford dealer. Now 2013 there already is the bubbles in the mint fenders
you all know where, at the split line, bottom near running boards. The truck
absoultly isnt driven in rain or snow, kept inside heated shop, its a service truck for onsite water plant. Ya we give it a wash job now and then. There ya go.
whats up with that?? Take it to bed with me? Move it to Arizona? Our shop
driveway or road is shared with the water plant which is new (asphalt) no need to even be registered (more like private prop.) the truck works behind closed
gates like for welding and so it never comes out of second speed. And we are
quite far from the ocean. ??????????????????????????????????

 

if you have a local PPG paint store around, they carry seam sealer in a yellow/silver tube. i cant remember the name of it off the top of my head, but it works great and you can paint over it with any epoxy paint. i have used it for the last few years and havent had any issues.

 

That's honestly a sorry looking reinforcement. Guaranteed to collect dirt and water. Maybe it's too late, but I'd be inclined to take it off the fender, form it to fit tight, and weld it up tight. Maybe put some weep holes at a minimum. Or remove it completely and use washers to reinforce the bolt holes, and weld a strip to reinforce the lip. THEN use the products listed. JMO....

 

Yup, that was what I was thinking. A single layer is much easier to prime and paint both sides. I even considered making my own patch panel out of 14 ga. but my sheet metal skills are not very good, that's why I bought the patch panels. Anyway even if I weld the perimeter there will be holes where the running board bolts are and water will wick in leading the problem that bigjob described.

 

I think some rubber washers on the running board bolts would go a long way toward keeping water out of the holes. Dirt that collects in a pocket is the worst, it holds the moisture in there. There was a discussion a long time ago about making the holes oversize so movement between the running board and fender wouldn't rip the metal, which happens a lot.

 

Well, yes I did go to the their web page. I appreciate your advice and just want to understand what and how this stuff works. The product data sheet (see image below) is brief to say the least. You say it can be applied over epoxy primer but I don't want to use it if it will dissolve my acrylic enamel paint job. The product data sheet says "contact our technical department for installation protocol." It also mentions an instruction manual but haven't found that.

 

yup, was thinking about rubber washers too. It's not hard to imagine this connection moving around a lot. I also considered making both patch panels (the lower section of fender and the rear flange of the running board) out of stainless steel. I've used weld thru primer in other concealed spots but I don't think it will hold up here. I worry about the seam between the fender and cab or rear fender and box too. I doubt the rubber welt is keeping all the water out but at least those joints are single thickness, have epoxy primer, and probably won't move around.

 

It will not affect your primer, it's solvent is a soddard solvent.
The product is a micro wax based, no acids, etc.
The manual is for application of a complete vehicle, same as the one that Chrysler issues for their dealers employee's for installation. (this application is the one shown in the training video on the site)
You can NOT paint over the rust inhibitor, paint won't stick to it, the "undercoating" they provide will adhere to it however.
School Bus maintance barns across the nation use the rust inhibitor and then apply the undercoating over it to increase protection from rocks and other abrasives that are produced when the buses drive on gravel roads, etc.
Just follow the instructions in the Ford documents on the website for correct application following a repair.
When Ford and Chrysler did their testing of the product, part of it was "creep" on bare metal and painted metal.
It had to "creep into and coat" down in seams that were tightened to a spec torque.
Another test was "water displacing capacity" where it had to displace any water present and coat the surface so water could not attack the surface.
In case you wonder how I know all this, I retired from the company 5 years ago and was the one that worked with Ford, Chrysler, GM, Hyundai, Kia, Mazda,Mitusbisi and Nissan regarding paint, trim and corrosion concerns.
I was also the one that worked with I-CAR on corrosion control, helped write the training programs, etc, and conducted, at their request, every two years, a seminar on corrosion control at their international meetings.

 

Here's MHO: Bolt it up and apply a paintable seam sealer around the edge. I haven't had any sealed seams ever rust. The original metal was not sealed, was mistreated, never waxed, left out and neglected in a field or salvage yard for 25 years, etc and lasted >40 years before rust out occurred. Are you REALLY going to be concerned if it rusts out again some 50 or more years (assuming better care than originally given) from now? Are you likely to be around 50 years from now??? By then the future owner(s) will likely be able to order a 3D printed or materialized brand new replacement, and be asking their future enthusiasts how to avoid buying a truck with patched panels! LOL! Build your truck for you and for now, not for the future, drive it and enjoy it.