I have a spare tank in good shape, has some mild rust inside though. Has anybody tried a slosh sealer like POR15...I know it's for gas tanks, wonder if it would work with coolant to prevent further rust?
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Maybe partially fill it with clean pebbles or nuts and bolts and shake to loosen the remaining rust, flush that out, then a rust remover solution, then the slosh paint (though I'm not sure if POR15 right out of the can is appropriate).
I've soldered up a pinhole here and there, but there's always more waiting to pop through.
Would having it boiled out at a radiator shop be a good starting point ?
Larry
Yes boiling will work. I just wanted to try and do something myself. I like the rattle with nuts and bolts idea. Simple, and no expertise needed. 
I too would like to know if there is ANY internal surface treatment(sealer) for the coolant tanks and tubes
This looks like the ticket for sure. Time consuming, but effective.
https://www.kbs-coatings.com/tank-sealer.html
How to videos (motorcycle one closely matches our coolant tanks) and is impervious to coolant.
Rust in the two tanks is so common, Ford/DeTomaso nickel or chrome plated them inside and outside on L-model Panteras. Remove the black paint to see if yours is plated. POR-15 may delay the inevitable. The big water pipes are mild steel (exhaust pipe) and commonly rust through near the ends. The fix is a good grade of aftermarket stainless steel from the vendors for tanks and water pipes. Paint if you're into the stock look.
A peculiarity of Ford-era Panteras is, one undercar pipe is two piece while the other identical pipe is one piece. The long pipe cannot be removed except with the entire powertrain removed first, or by cutting it in two to match the short assembly. I wouldn't waste time nor effort with refurbishing stock water pipes. I also recommend discarding all stock multi-piece Italian hose clamps and Pirelli rubber hoses before nature does it for you on the road....
As an update on this subject. The KBS coating will not handle heat, so it's worthless on these tanks. I'm convinced to just replace them with stainless. Even if you get them cleaned out they will be thinner, then the whole process starts over again, meanwhile you are getting lots of rust particles circulating in your radiator and blocking it up! Not worth messing with them.
Absolutely agree. I boggles the mind what kluges are put together on a $75k sports car to avoid spending what it takes to actually fix a problem. Typically, the connecting hoses are shot as well as the caps. Somewhere around here I have a photo of a Pantera surge tank with the whole bottom replaced by a soldered-on tin juice can painted black! Some vendors have usable stock take-offs from people into bling, as well as new non-corroding stainless and aluminum tanks, both with new caps.
any ideal which Vendor makes their own?
Pantera Performance does a certified fuel cell for around $ 2,500.
JFB, I doubt if ANY Pantera vendors actually make their own tanks anymore. Pantera vendors are one-man operations these days The tanks are billet weldments so they are likely subcontracted to a small certified weld shop, who make up a dozen or so at a time to order.
I just ordered mine from Hall. Reasonably priced.
BTW the overflow tank will rust faster since it is partially filled with fluid and is hot and steamy in there, perfect conditions for rapid rust. At the very least get a magnet stick and sweep the bottom every once in a while. You may be shocked at what you pull out of there. If you ever look inside with an LED stick light it will frighten you!
I was just thinking about if the Vendor could have the overflow tank mod'd to convert to pressure tank versus paying someone local to make the additions.
I guess any good body shop welder should be able to put in the extra pipe barbs
