The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.