The following pictures may be of interest - this is a steel boiler washout on a 3 inch scale traction engine this weekend.
Unfortunately there aren't many places to use a water jet on these boilers and so I tried a new approach (for me) - a 24 hour soak with a product called Metal Rescue which seems to be widely available in the US. The stuff isn't cheap, around $26 a gallon from Home Depot, and the boiler holds 3 gallons.
I bench-tested a small quantity of the stuff beforehand by soaking some steel, bronze, copper and brass components to check that it wouldn't attack anything other than rust, it seemed to work as advertised so I went ahead and gave it a go.
During soaking the liquid turned from completely clear to roughly the color of old sump oil. A lot of entrained rust particles came out with the liquid on draining as well which was a good sign.
After draining the boiler was thoroughly flushed with clean water - filling the boiler fully with water and then stop/start draining from both front corners of the mud ring until the water ran clear and there were no more particles being flushed. That took eight complete fill/drain cycles. The boiler was then thoroughly dried as usual.
In summary the wash seems to have been successful - a lot of rust was removed and I can now see exposed bare steel on the tubes using a boroscope that was certainly not visible before. The fusible plug began leaking during the soak but this was long overdue for replacement. I have yet to remove the plug to confirm, but strongly suspect that there was a pinhole in the plug that was blocked with rust particles that were dissolved away by the product.
The setup:
Before (longitudinal stays, firebox crown):
