i tried soaking the lumps in different liquids such as methanol caustic etc but none softened it,in the end i got a peice of 1/2" plastic tube,and then a length of s/s 5mm strainging wire about 6" longer than the plastic tube,frayed one end and put the othe end in a compressed air drill,and worked the pipe around inside the tank,it removed most of the loose stuff,and then filled the tank with rust treatment for 2 days,still some big lumps inside but they will come out in time,and i think thats maybe whats best for you,give it time and rince regularly,and most of it will work itself out,some owners are to keen to seal their tank,when the tank is repairable.