I am restoring a 5" Duchess the boiler of which is in generally good condition. However the flue tubes, which are about 15/16 ins. I.D., are coated internally with a layer of hard, black carbon. I have given it the usual flue brush treatment which produced clouds of black dust but a hard layer about 1/64 ins. thick remains. I have tried fitting the flue brush into an electric drill and giving the tubes a few minutes of rotary action to little effect. I am wary about trying anything more aggressive as I have heard quotes of over £2000 for a replacement boiler. Any ideas anyone? Barrie Purslow Warrington UK MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.