No problem, it's a good question! Actually, the first tank I was given was seriously rusty, inside & out, as it had been left open, out in the rain, for several years. Even that wouldn't have been a big problem normally, as I intended to take it to a local radiator/fuel tank place & have it boiled out and coated. When the guy took a look inside, though, he found someone had put a patch of tarry stuff in there, and no method (including some exotic/scary solvents!) short of cutting the tank in half & grinding it out would remove it.
So, I found another guy locally who had a tank that he'd just taken out of his Spit, to replace it with one of the Ebay aluminum ones. He gave me his old tank (plus a bunch of other bits!). This one I took to the fuel tank guy, and he cleaned it out completely & lined it with sealant (guaranteed forever, free repairs if ever required). Then I sanded & repainted the outside, and it's now essentially a brand new tank.
Interestingly, the tank I ended up with is baffled, into 1/4-1/2-1/4. It's the only internally-baffled factory Spitfire tank I've ever come across, yet it is an OEM Triumph unit.
In my case, since I'm using Smiths gauges, the Spit sending unit works directly with my fuel gauge.
In the end, though, if the Spit tank idea works for you & you can't find any decent ones locally, you might try Ebay for the welded aluminum ones. It still costs more than doing what I did, but they really are nice.
By the way, the Spitfire tank fits the rear of a Locost like it was made specifically for it, better than even the JAZ plastic tank I was considering. Mounting is a snap, access for the fill/fuel out/venting etc. are right where they need to be, etc.
If you find an original that's still in a Spitfire (and is under cover), chances are the tank will be pretty good inside.