I had to countersink the cover for the tinnerman washers and the lip for flush nutplate rivets. I didn't want to use a metal countersink because the fiberglass would pretty much ruin it and dull it almost instantly. I remembered reading about an abrasive 100 degree countersink cutter awhile ago, so I started searching. I found it! The permagrit 100 degree countersink. I searched high and low for a supplier in the US, but no joy. I ordered it from Perma Grit in the UK. Shipping wasn't bad and didn't take very long either, I think about 5-6 days. Wow is this thing worth every penny of the 25 bucks it cost. Beautiful countersinks over and over. I opted for the “no nipple” because I wanted the ability to countersink different diameter holes, which worked out well. If you are even thinking about countersinking fiberglass, just get it!! Anywho, I installed the washers, nut plates and screws. Closed everything up and boy does it look good. It's a lot of screws, but it definitely will not come off haha. Oh and what a surprise, Vans just emailed me and told me the wings will be here in a week!!!!! Way ahead of schedule.