When working in marine industry, we repaired dozens of tanks as corrosion was common. There are plenty of ways to repair classic or vintage tanks….. just not in the airplane world as our stuff is somehow special… it’s not.
there are process of internally cleaning tank and then spray coating it with sealer that will out last the plane……doubt it’s prior FAA approved, but A&P has wide discretion of proving stuff for vintage birds.
Any decent fab shop, especially one doing old cars or boats, can cut out parts of tank and weld in patch…that’s FAA approved method, but a bit extreme for just small areas of surface corrosion. They can also separate the tank along seam and you can repair and/or recoat surface, then re-weld it together and pressure test it. I don’t expect cost would exceed $200 to open and reweld back together. This, of course, requires tank be removed, which may be hardest part on old airplane, but doable…..likely do it yourself. In my experience with this,frustrating part is small parts like the skin or tank grounding straps or tie down strap fasteners can be difficult to remove and can be damaged (such as screw heads breaking off) so be ready to drill some screws out and have replacements. Would also suggest spraying areas with long term anti corrosion spray.
If there are multiple points of corrosion, this would be my method of choice…. You could either sand/grind out areas and seal surface yourself , or cut it out all together and weld a patch…..either way, repair will out last the plane.
Again, there is FAA AC for wider approved methods of keeping old aircraft airworthy. Most A&Ps don’t do this stuff anymore and simply source another old part from a salvage yard….which is same age as yours. These vintage tanks are all in bad shape…. Many domestic companies make replacement tanks for vintage cars….you could ask them cost to replicate your tank….it would be possible to get a A&P sign it off, especially as vintage, but you could also get it FAA approved or at least a field approval …..the field approval could be shrared with others if you want to get into selling tanks to other in same situation. I know there will be post saying you cant use a field approval to sell parts to others as method of compliance, but FAA does allow wider methods of compliance and approvals for vintage aircraft…. You may need some additional engineering data and technical mumbo jumbo, but it is a simple part….. I could help you in this regards to advice with this, PM if needed. I expect sharing specific field approvals will be allowed in near future for vintage. Could be a way to offset your tooling costs and make minimum run quantities. I worked on such projects as a DER, and it’s possible without as much start up cost you may think.
Anyway, if you try to keep old planes flying using only strict approved or certified methods, you will go broke. This stuff was made using 1940s tech, materials, and processes….same as car parts, it’s nothing special. The TC holder includes approved methods for repairs, such as a structural repair manual, outlining repair methods,in their type design docs, but you can get other methods approved, especially when new parts no longer available. Good luck. I have used many unapproved but new parts on my 1969 c172 and they work fine, but it has an experimental AWC, so I don’t have todo as much FAA approval stuff…. If I did, and had to pay the certified costs, I couldn’t afford to keep it.
Again, good luck, may the force be with you
