If the bottom of the upper wing skin isn't painted, the aluminum oxides rub off on the clear tape. Fortunately, they clean off fairly easily.
Try painting the bottom of the skin. It won't show and should fix the issue for next to nothing in price.
Yes, you can put teflon tape, or other, under that skin, but aren't you trying to protect the flap paint? Any grit caught in the underside of that skin would scratch the flap paint, hence our recommendation to protect the flap with the paint protection film.
Or do all of the above!
F1 Rocket and F4 Raider components
RV and Rocket Accessories, Tailwheels, Tools, & More
F4 Raider - under construction
F1-H Rocket "Crazy Horse" - sold
RV-4 "Chief Pontiac" - sold