As you can see the diesel stain your tank is probably opaque, is it UV protected? If it bothers you change it to a black UV protected tank. I wouldn't be bothered about the diesel stain as such, I would be more concerned with the the integrity of the tank.
Disconnect the diesel line and plug the track outlet then fill it with white vinegar for a few days. If that doesn't work try a strong bleach solution. If that doesn't work forget it and concentrate on something important.
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"I beseech you in the bowels of Christ think it possible you may be mistaken"
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Paint probably won't stick to the type of plastic. Whitworth Marine sell 5, 10 & 20L black plastic ones. I have some 5 & 10L & they are very well made.
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Procrastination, mankind's greatest labour saving device!
50L custom fuel rack 6x20W 100/20mppt 4x26Ah gel 28L super insulated fridge TPMS 3 ARB compressors heatsink fan cooled 4L tank aftercooler Air/water OCD cleaning 4 stage car acoustic insulation.
Most diesel heater fuel tnaks are made from an engineering plastic such as acrylonitrile butadiene styrene (ABS) or similar which is also used for plastic car bumpers etc.
This can be painted with appropriate preparation.
