What can I do if my wetback develops a thick coating on it?

The wetback can develop a coating of crusty creosote when the wood fuel is not being burnt in the most efficient way. Firewood can play a major role in the performance of a wood fire. The species is part of the picture but the most significant thing is that whatever the type of wood it must be well seasoned and dry. Best performance cannot be achieved without the best fuel.

So back to the question...

Burning wood at low temperature causes incomplete combustion of the oils in the wood, which are off-gassed as volatiles in the smoke. As the smoke rises through the chimney it cools, causing water, carbon, and volatiles to condense on the interior surfaces of the chimney flue. The black oily residue that builds up is referred to as creosote, which is similar in composition to the commercial products by the same name, but with a higher content of carbon black. Over the course of a season, creosote deposits can become several inches thick. This creates a compounding problem, because the creosote deposits reduce the draft (airflow through the flue) which increases the probability the wood fire is not getting enough air to burn at high temperature. Since creosote is highly combustible, a thick accumulation creates a fire hazard. If a hot fire is built in the stove or fireplace and the air control left wide open, this may allow hot oxygen into the chimney where it comes in contact with the creosote which then ignites—causing a flue fire. 

The easiest way to clean the flue is by placing a deep baking tray or similar under the base of the flue and sweep the flue down into this. This stops all the debris from falling into the top chamber and requiring vacuuming out. The build-up around the wetback is best removed by hand and the rest can be carefully removed by a vacuum cleaner.

The wetback can be knocked out of alignment if it is moved when the creosote is being cleaned off. This can cause the constant rise to be knocked out of alignment and can result in water hammer developing in the system so be careful.