Back again
First off, NO rubbing alcohol, use denatured alcohol available in most hardware stores. I guess they have those in the british islands
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To the fins, yes cover them off the plane. Insert the un-covered fin in the wing, with a pencil and light pressure mark the fin were it meet the wing (both sides). Remove the fin from the plane and mark a line 1/16" to the inside of the wing on the fin. This is were you want the covering to stop.
Many people will cover the entire fin first and then mark the line and cut the covering with a razor and remove the waste. The problem with this is it that it is near impossible not to cut some of the balsa while cutting the covering and that cut is at the point were the highest stress is put on the fin. you can guess what can happen.[>:]
Now to the control push rods. If I remember correctly Ny-Rod is a trade mark name for one of the Sulivan nyrod products. The trick to ny-rods are to anchor the outer cover, it need to be anchored ever 3". any movement of the out tube will translate to slop in the control serface. Like you about all I'd ever use them for is throttle control.
That being said, I haven't used a ny-rod in years for control surfaces, I just don't care for them. I put one servo in the skin of each wing straight forward of the control serface control arm and run elevons. A little less weight and no slop.
A lot of people are scarred of elevons. They say if you loose one servo you have no control but if you loose one servo with one on the elevator and one on the ailerons you still pretty much hosed.
The biggest trick here is good equipment, and old standard servo that has been in 5 airplanes and that you've had since you got started in the hobby probably wouldn't be your best choice for a servo. Best control of course would be a digital servo but any duel bearing coreless with 60+ oz should work fine.