My "non engineering" comment is..... A similar airfoil was used on the PIK-20, SGS-135 and others.
When clean, it worked well.
When dirty (bugs, dust, rain) the bottom dropped out.

Common cure was:
-sand to 45* of chord with 600 grit
-wipe D-tube with liquid dish soap and let dry

Sanding allowed a turbulent boundary layer, soap sheeted water, thus killing drag!

Long final glide in a "20" with others into Mifflin, a 1-35 (highly modified) showed the drag gain when we hit rain showers, he had a tough time, we made slight adjustments to final glide.

So, that airfoil can be good clean,but really sucks when dirty. No, you DON'T want a high gloss finish!!!