Printing material for methanol tank
Most commercial blow-molded fuel tanks for model airplane fuel (methanol or ethanol, nitromethane or nitroethane, and some combination of castor, mineral, or synthetic lubricating oil) are made from HDPE.  This material isn't commonly seen as filament, in my limited experience, but it ought to be possible to arrive at settings that will give a liquid tight tank without further sealing if you can find some.  As you note, limonene might be used to smooth/seal HDPE prints, but likely won't be necessary if your settings are right.
You might want to test PETG filament for its resistance to your fuel mix(es) -- this material is available as filament, prints with settings little different from generic PLA (in my experience, higher nozzle and bed temperature, and a little more bed clearance for the first layer), with good layer adhesion and, with a good print, is liquid-tight as printed.  It's not particularly flexible (as is the case with HDPE), but since you can customize the shape of your fuel tank, it may work for you -- or it may be more flexible in vase mode, as PLA is.
Sealing PETG may be as simple as baking it (similar to &quot;heat treating&quot; PLA to increase print strength, albeit again at a higher temperature) -- this partial remelting will ensure that layers are adhered throughout the print, which (presuming you have avoided under-extruded areas) should be all that's needed to make a printed tank liquid tight.