Thanks AB.
I did a bit more Internet digging and you seem to be correct. At least everything I read indicated that you should empty the tank after use and drain. I just dumped 20 gallons @ 150 PSI, then opened the petcock and drained a small amount of water. Interestingly when I tried to close the petcock, it felt like something was grinding in the threads. So I removed it and it would seem that when it was manufactured they forgot to deburr two holes and their sharp edges were catching on the inside. A quick file and it works better than new.
My digging also landed me into videos regarding the air delivery system. This is one area that you can spend quite a bit on setting up for HVLP...a lot more than the few hundred I have spent so far. So I am going to make some changes, which will hopefully improve my results.
Some easy ways to improve the performance of my low-end system is to address the volume of air it delivers. Apparently when it comes to HVLP, the biggest factor is volume of air, at least more than air pressure, which is also a consideration. Sounds obvious in looking at it now, hence the “HV” (High Volume) in HVLP. Strangely enough when I solicited advice before the emphasis was on pressure, as in the “LP” (Low Pressure)…I know, don’t say it!
Besides the guns, which can get quite expensive, I now know that my fittings, connectors and airline are too restrictive for the optimal CFM, and these are relatively inexpensive to swap. So I will be swapping them for 3/8 ID. Also the water/air filter I have is very cheap and likely doing very little except adding to robbing performance, so I will be looking at my options.
One other area I will look at and may or may not do anything about is adding some vertical loops of pipe after the tank with drains at the bottom of the loops to drain any accumulated water. This apparently helps to cool the air before it hits the filter, but I am not convinced at this time that this is something I need to do for hobby purposes
As for the compressor, it looks like what I thought was a big tank, 20 gallons, and a big enough compressor, 1.8HP, it is possibly undersized. The general consensus is a min of a 60-gallon tank, 5HP compressor and a two-stage air compressor. However these numbers seem to be the optimal setup for painting a car, and not a relatively small hobby rocket, so I am not as much concerned with the tank and compressor size at this stage.
I will see how things progress after the airline and fitting change and if anything I will likely upgrade my gun, or add a gun specific for top and clear coat. This is because gun I have is really optimized for primer.