Low polygon count glider... LOL love it! Yeah this is now a HD Spectra *grin*
Yeap that's some evil black funk inside the fuselage. My Apache balsa DLG spent a night out in the rain (long story), got a little wet. I cut a few holes in the covering & let her air-dry for a few days. Almost immediately I started seeing that same black stuff here & there. I also had a balsa Puddle Master flying boat that got wet all the time. That too had areas of the dreaded black lurg. This Spectra is 20 years old so I'm guessing it's seen its share of moisture over the years. Before working on this project I diluted some bleach & ran a sponge over the scary areas. While it's not very attractive it doesn't seem to harm the structure any. Just one of the joys of working on an old airframe.
Speaking of early electrics, the biggest problem I had with my first e-gliders was the fixed propeller. Folding props were expensive & pretty rare back then so we just bolted on a regular blade & went flying. I recall making low passes, so I could see if the prop stopped in a horizontal position for landing. Broke plenty of props back then LOL.