Im by no means an expert but i have just done the arches and they look really good, a lot better than i expected anyways.

Feathering in is a paint technique it basically means that you move away from the paint when you are past your scratch if that makes sense, to save putting a thick coat of paint all over the area that will leave a line and then have to be rubbed back.

Imagine when a wave breaks, it gently gets shallower and shallower when it hits the shore... a shit analogy but it kind of sums it up lol!