This was a picture posted in the facebook Kitfox group and is the reason why you do the polybrush coats on the top of the wing tanks as described in the build manual. The polyfiber manual also talks about precoating plywood surfaces with polybrush before covering with fabric. I see the top of the tank as a similar issue to a solid plywood wing like a Bellanca Viking. A note that one of the posts in the Facebook post mentioned that Dan Denny recommended using a 50/50 mix of polybrush/polytac instead of just polybrush. I have no confirmation of this last bit.

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