I have recently purchased a 'Maid if Kent', and steamed it for the first time today. The tubes are 7/16" but I find it only possible to clean 2 of them with the correct size flue brush.
On inspection the tubes are coated with a layer of hard soot, easily visible with a torch, it starts about an inch or two from the smokebox and prevents the brush going down the tubes. At a guess about .010" thick.
How do I go about removing the layer to improve steaming (heat conductivity).
Eric Penn
Boiler tubes
Soot
Hi Eric,
Plan A.
Try to soak and dissolve it with a stove window cleaner. (=some kind of petrol solution).
If it doesn't do the trick go to plan B.
Plan B.
If a brush isn't effective
, try to make a copper scraper. Don't use steel otherwise you possibly demage your tubes. 
It could look like a copper disc of 7/16" fixed on a bar. Drill holes in the edge of the disc to create "teeth".
By moving it forward and backward while turning around, you should remove the soot quickly.
And if not then you have to drill it out very carefully.
Frits
Plan A.
Try to soak and dissolve it with a stove window cleaner. (=some kind of petrol solution).
If it doesn't do the trick go to plan B.
Plan B.
If a brush isn't effective
, try to make a copper scraper. Don't use steel otherwise you possibly demage your tubes. It could look like a copper disc of 7/16" fixed on a bar. Drill holes in the edge of the disc to create "teeth".
By moving it forward and backward while turning around, you should remove the soot quickly.
And if not then you have to drill it out very carefully.
Frits
Build it perfekt, but keep it simpel.