OO9 is 4mm to the foot scale (that is OO or 1/76) but using 9mm track gauge (the same size as N gauge). This is...
It is best to varnish the whole model. By coating the whole model in varnish, you are guaranteed a uniform finish across the entire model. I do not think that it is possible to apply the exact amount of varnish to the area that is going to...