Walllllldo...
The fabric at the top is most likely "rock wool", a type of insulation put there to keep the outside of the heater from getting too hot.
If you've cleaned (or had someone else clean) your jets, then they may have not been done correctly and made too large as a result, which will allow more propane through than was supposed to, so that it burned richer than originally designed.
Once the coils soot up, they need to be cleaned. This is because the soot collects between the coils- and specifically, the upper layers known as "pancakes"- and interferes with airflow. When airflow through the pancakes is interfered with, so is airflow through the heater (i.e., the combustion chamber), so that less air is present- resulting in the heater burning "rich" (i.e., incomplete combustion, resulting in soot).
So, when the coils are sooted, it starts an accelerated cycle of more soot.
As a result, it's important to:
1.) Check your jet size and burn (basically, what Larry said to do)
2.) Clean the pancakes (blow them out thoroughly with compressed air)
3.) In addition, if you haven't changed your gas valve in a couple of years or so, change it out as well. The perfume that that they put in L.P. to make it stink tends to build up over time on the inner parts of the gas valve (and regulator), and interfere with efficient flow- through.
$200 in parts and about an hour of work and you should be good to go.
