Thanks for the reply. The diesel tank is a section of the keel. When built a polyurethane
coating was flow coated over the interior
. There are only four holes:- (a) 12mm vent (b) 38mm filler (c) 8mm outlet (d) 12mm drain. I have just started to get very small amounts of rust in my filters and this is after 30 years of usage. I want to remove the diesel and replace the diesel with sump oil to prevent further deterioration. I want to reduce the volume as much as possible so that I am not carrying around an additional 250kg (approx) of oil. I have installed new diesel tanks
. I can pour the foam in small mixes through the 38mm filler. I have had conflicting advice about using foam. Some say that the oil will not affect the foam and others say that it will. There has to be another product that is not destroyed by the sump oil and would be able to be used as a filler e.g. table tennis balls etc. There is absolutely no need to cut a large hole in the tank to achieve the results that I am trying to do.