I am in the process of restoring a Aster U232 U1 which had previously been coal fired and substantail scratching to the left hand side of the boiler and tender. The amount of mess left over from the coal firing is amazing and I'd like to use the opportunity of having it in pieces to clean it up. Worst stuff is inside the smoke box and at the front end of the boiler and I can't find any solvent that will shift it. At one level, it's all very authentic but I also worry that this coal/oil residue could be corrosive when wet. Any ideas?