I'm sorry, but trying for a zero gap works likes gangbusters.

Do I ever get one? Have you ever? Has anyone? Sorry to have caused you concern, but I should have typed a lot more words and described what I do in detail. I actually try for as close a gap as inhumanly possible and almost always get it. I also will use nylon mechanical hinges as my preference because they will have almost zero resistance to the surface movement yet are almost bulletproof in strength and wear. And whenever I can, I also use wire in metal tubing along with those hinges. If I have a steering tail wheel that's driven by the rudder or if I have torque tube driven ailerons, there will be wire that does the driving and with some design effort, I can run the wire through a tight fitting tube. The tube will then be attached to one side, and the wire to the other. It gives a massively strong hinge that is awesomely free of resistance. The sucker always happens to be quite a different dimension compared to the nylon hinges and takes some craftsmanship to have the different thicknesses built into the two surfaces so that they have a common hinge line. And that works like gangbusters also.

As for zero hinge lines actually opening up or whatever it was they were supposed to be doing wrong. I just came back from the flying field where just a couple of hours ago I was holding up my Hots against the setting sun so the guys could see that in fact, you could see light along the hinge line. It happens to have wire/tube hinge, nylon hinges and "alternating crossed fabric" hinges all on each of the ailerons. And it was built in 1980something and is into it's 3rd engine. I tried for zero gap on that airplane and darn near got it. And it flies just awesome. And after a lot of real hard use, you still have to hold that baby up to the light to see through the hinge line.

BTW, almost all nylon hinges and any one made with "hard" materials will have some tolerance that allows them to allow the hinge line to open up. That is, if they are installed very nearly "square" to the hinge line. So I don't. I alternate them being about a degree out of line. And that way I can get the movable surface to stay almost zero distance away from the surface it's hinged to. In theory that's not a good practice. In practice it's an excellent practice.

BTW, my practice of putting a couple of drops of CA on one side of a hinge and then hitting the other side of the same hinge quickly with a couple of drops also is an excellent practice for me. I guess I don't understand what could be wrong with it. It certainly has insured that I wasn't forgetting to glue both sides of any hinge, and it's insured that both sides have a chance to wick before the CA applied to the first side kicks and seals just a small area of the other side to future wicking when you try to apply CA later.

You know, I have noticed that for any building task, there is almost always way more than just one technique that works well.