I saw a few threads about people talking about this. How do you do it and why?

There are several reasons why. The two most popular are that it can lessen your chances of creating flutter which can damage the plane, but mostly to stop air from flowing from one side of the control surface to the other through the gap. This greatly increases the effectiveness of the control surface.

how does a feller go about doing this? and with what materials?

You can use anything from a strip of covering to cellophane tape. It is usually placed on the underside to hide it. fold it into a "V" shape, deflect the control as far as it will go (disconnect push rods first) and adhear it to the gap area.

See picture:

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Mike, since you have all these neat graphics maybe you could do them in a series with an explanation and we could just reference the thread to everyone when they ask. This seems to be one of those weekly sometimes daily questions.
Ed M.

Ed, I think you just gave me the idea for next months "How To" column.
Thanks

Ed, I think you just gave me the idea for next months "How To" column.
Thanks

MinnFlyer.....it was nice meeting you Saturday - and I love your diagrams = with color no less!
Jerry

A good material to use is Blenderm medical tape. 1" wide. It sticks well, is transparent and has some 'stretch' capability.

Do not use any medical tape that is perforated (like Transpore), as it can easily split.

Craig.

Nice meeting you too Tailskid!