Decided to not use proseal. With bottom side down, weighted flap down with about 130lbs potting soil and sand bags on top of an eight-foot long 2x4 to distribute the weight. Essentially the same thing I did when riveting the flap prepared with proseal. Inserted rivets and taped for back riveting. Turned flap top side down and again weighted it down with 130 lbs. Had to adjust some to get all rivets to fit through holes. All this was done on a eight foot long flat table with a bucking bar consisting of a ΒΌ inch by 1.5 inches by 36 inch steel bar set flush into the middle of the table in such a way that it would permit laying the flap bottom side down with the hinges off the end of the table yet with the trailing edge over the middle of the bucking bar. With the top side down I half way set the shop ends of the rivet working from center to end using a back rivet set. Then I flip the flap over and set the rivetsg a mushroom set. The result was a perfectly straight edge. I did some minor adjustments using the mushroom set to close some small gaps at some rive locations. Based on this experience, I would not recommend using proseal before riveting.
