With the help of my friend Jim, I drilled the flap to the right wing today. I began by removing the flap hinge and trimming about 1/16" from the depth of the "wing-side" of the hinge. This is so the hinge will completely clear the 45-degree bend in the flap brace.
Next, I un-clecoed and removed the lower outboard wing skin, temporarily pop-riveted the flap brace to the spar in five locations, then carefully positioned the flap in place and clamped it from each end, making sure the holes in the skin and flap brace lined up, and the edge of the hinge lined up with the reference marks I made previously. I taped the flap in the full-down position (90 degrees up, since the wing is upside down on my table), then drilled the holes through the skin, brace and hinge. Since it's impossible to get side-grip clecoes, or any other kind of clamp, near the hole being drilled, it helps to reach underneath and push up slightly on the hinge, while an assistant pushes down near the hole being drilled. The assistant can also drop a cleco in each hole as it's drilled.
After drilling the flap hinge, I removed the flap and hinge-half, then deburred the hinge and skin. I drilled out the pop-rivets holding the flap brace to the spar, countersunk the brace, and cleaned and dimpled the row of holes in the skin.
I primed the aft surface of the spar and the lower surface of the skin, then painted them both white. Finally, I painted the inside surface of the flap brace white. (This triangular area will be almost totally inaccessible after it's riveted together, and I wanted some lasting weather protection in there.)
Photo 1: Drilling the flap hinge to the wing. The blue tape near the drill protects the skin from the chuck.