I used my last metal fdt gas tank setting on the shelf last summer to replace a leaker. Metal ones in good shape are getting harder to find so I purchased a well weathered Lawn-boy Commercial tank for a decent price off a FB page. Read about others using them here. https://simpletractors.com/forums/topic/46259-fdt-plastic-replacement-gas-tank/?tab=comments#comment-83318

Tank looked like this when I received it. Well actually I'd already cleaned it up when this picture was taken. May have been setting under trees in a mower graveyard somewhere.

So I tried to clean the black spots off w/cleaner but no luck. Today I took some fine sandpaper and rubbed lightly. 

I'll probably paint it when I put it in a tractor (AC yellow or Simplicity orange) so the scratch marks should help the paint stick (I hope). Cleaned up nice: