All glider pilots would use some sort of electrical tape, or similar to fill the gap between wings and fuselage. All the ones I know that is
I thought about it, but I think I'll sort out my undercarriage drag first.
Also, some gaps migh be better filled with building foam, sanded and filled with body filler, then painted. Door gaps for instance. I haven't done it, just an idea, so don't take my word for it. The door's edge might be covered with packing tape as protection and a release agent.
But the above is only an idea, haven't done it yet.
Is there any pics of your project aside the one you have in the signature?