Some thoughts on About Time2's tank cleaning:
1. Dont waste your time and money on 'fuel polishing' a dirty tank, especially if you cant get inside to either 'steam' power wash or hand scrub. So-called fuel polishing only cleans the oil not the tank walls.
2. On a blind tank that you dont have an access port, etc., fairly 'decent' results can obtained by using an enzymatic chemical tank cleaner
http://www.amazon.com/Star-Brite-St...=UTF8&qid=1420640682&sr=8-8&keywords=startron
With such a product you wont get 'all' the crud out but most of the 'deposits' will be dissolved, thus lessening the potential of their 'breaking loose' from the surfaces during heavy sea states. Long term soak is best with an 'enzymatic' and will 'stun' / kill and then 'dissolve' the microorganisms and their 'products'. After treating with the enzyme, Id then recommend to drain the tank and take the residual home and burn it in your home oil burner. Id then repeat the process if there is a LOT of 'discolored' oil when you drain it. Keep repeating until the tank contents begin to remain 'clear'.
Obviously youd need to have spare 'filters' on board just in case some of the 'residual' breaks loose. The typical sailboat sized Racor, etc. only has about 25-40
GRAMS of 'dirt capacity' before plugging - a trivial amount before 'plugging'.
3. for a small tank such as this, consider to remove it and either instal a 'hand hole' and clean it yourself or send it out to be ultrasonically cleaned ... look for a 'radiator service' who caters to big 'trucks'.
Caution - for gasket material for any 'access ports' in fuel tanks should probably be Viton® rubber, due to the increasing 'mixes' / blends with 'bio-fuels'. Increasingly BUNA and Neoprene will soften and easily deform in the presence of the organic acids that are present in so-called 'bio-fuels', especially 'reclaimed' bio-fuels.
4. if you do any cutting while the tank is still in the boat, first empty the oil, fill it with WATER when you do any metal cutting - a safety precaution. Can prevent a loud 'BOOM' while metal cutting.
5. metal cuttings - can be minimized by first boring a 1-2" dia. hole, then placing 'sticky tape' (I like StickyAss duct tape) on the 'underside' BEFORE you fill with water. Once you have a hand hole any remaining cuttings can be easily removed, manually with a soft cloth, etc.
Good luck. Hope this helps.