The ones we have been using are more of a clear plastic, but, yes, the pins are permanently installed. What we like to do is put a 45 degree taper on the back side of the wing and front side of the flap (same with stabilizer and elevator) where the "crease" comes right in the middle on the center line. (It thus tapers top and bottom to a sharp edge). You can simply glue quarter inch triangle stock to the back of your wing and front of your flap if you want. We did that at first, but Matt has now made a "tool" out of plywood with sandpaper attached by which he can sand it in.
Either method allows you to get an absolute straight edge down the wing, and it allows you to fit the surfaces accurately and right next to one another so there is no binding and no gaps. I know I will get gasps from some when I say this, but done right, you don't even have to seal your hinge lines. And if you do, it is very easy with the flat edges this way.
The hinge slots are then cut right in the center of this edge strip--right at the point. It is easy to hit, again, easy to do. Matt has gone exotic and now has a hinge slotting machine, but a simple slot cutter with an Xacto knife works well--just a little slower.
Now, for alignment: Cut a notch for the barrels of the hinges half way into each surface (halfway into the wing, halfway into the flap) so the hinge line is directly on the "point" of that edge. Cut, fit everything, then wait and finish the plane completely before putting in the hinges.
Once the plane is finished, you can epoxy the hinges into the wing first using slow cure glue. Put a little oil or Vaseline on the barrel, put epoxy on the hinge, and slide each one in its slot. Then use a straight edge and hold it against all of the barrels at once, making sure that each is touching the straight edge and that they are square against it. While you are doing this, you need to hold the straight edge with a gap equal to half of the thickness of the barrel on the hinges. You can tape a couple of thin spaceers down the wing between the hinges to help in setting this gap. Once all of the hinges are lined up on one side of the wing, do the other side and let them cure. Now you can apply glue to the other side of each hinge and slide the flap on right up to that fine edge (with the spacers now removed.) You will end up with a perfect hinge line, no gaps, no binding, no sweat.
And those 45 degree angles you sanded in the wing and flap make it easy to clean the hinge line when you are cleaning up the plane. It just seems to us to make sense all the way around. Ideally, you want to fit the controls so that they will drop, naturally, on their own when at rest. This means careful fitting of the pushrods in the horns and belcrank as well. But, any binding here, will come back to haunt you later on.
