Diesel tank sealant - Flowliner or alternatives?

if you have enough access you might try Raptor coat, it is resistant to pretty much anything, can be sprayed brushed or rollered and forms a very tough yet slightly flexible coat. It is so adaptable I once repaired a fuel pump diaphragm with it. You can also improvise rubber sheet with it if you spray it onto a waxed surface or masking tape.
Whatever you do with coating, make sure you dont block any holes/breathers. No reason not to coat the outside too. A four litre pack is about £70 quid, but you could do your whole boat in that. You need only a litre.
 
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