Rick -
Thanks for the compliments on my car. Sorry, but I think I'd notice if my Webers were gone - I've had them for about 25 years, and I am rather attached to them. Some nights, I go out to the garage and lift the car covers just to do "bed checks"
I collected about six FE tanks at swap meets while I was preparing my car. I picked the very best one out (no dings anywhere) and took it to my welder to have the brackets and inlet modified. When I picked it up, there were two big depressions in the top of it - I was sick, 'cause I had thought about polishing it, but that wouldn't work now that the top was no longer "virgin". Ended up painting it black.
I have not heard of anyone coating the inside to prevent leaks. I have done that to a fuel tank, but never a radiator tank. I guess I wouldn't recommend it for a water tank. Any radiator shop can easily repair a leaking expansion tank with solder - if it had a coating inside, it probably would have to be taken apart and stripped of coating before a repair could be made. On top of that, a cooling system is pressurized (where a fuel tank is not), and runs at elevated temps, so the coating may not prevent a leak anyway.
regards,
Jeff