Re: Cleaning a fuel tank
My usual method for small tank cleaning is: long handled scrub brush, loosen and knock all the 'crud' down to the bottom and sop up with copious amounts of paper towels and then burn the towels (where permitted). For the residuals, I dump in and spread 'high potency' garage floor cleaner and after letting it soak/dry, I use a shop vac to remove the 'solids'. Followed by 'sloshing' a minimum amount of clean oil to rinse the walls, sop up again with towling / polypropylene fluff (from oil absorbent 'pillows'), partly fill with new oil and then (and only then) continuous recirculation polish until 'crystal clear / no haze' when backlit with STRONG white light.
Was deeply involved in filtration engineering for ~35 years.
Cleaning fuel tanks is fast and easy (if you have proper access) ... just 'messy'. The long handled scrub brush is the fastest and with the highest 'efficiency'.
Most of the time the 'gunk' is a FUNGUS that uses the oil as its nutrient source.