I found a great way to mask an area where tape won't work - Model Railroader Magazine

My question is - how sharp are the resulting edges?
I can spread glue around with the best of them, but even the best of them tend to get an erose, ragged glue-edge.
Have you tried to obtain a sharp edge by trimming the glue (after it dries) with a sharp X-acto knife (using, say, the window frame as a cutting guide). If so, how does it look, and does it cut easily?
I bring this up because I have a bottle of 'professional' masking liquid (i.e. designed specifically for masking patterns on model planes and cars, and yes, trains), and even with brand new blades this stuff tears and pulls, giving me a useless, erose, ragged edge. Crud!