Plasterings not easy man. Theres NOTHING worse than a bad plasterer either (except a bad mason ). I was fitting a kitchen when a shit plaster had been in, The walls were so shit. I had to spend 2-3 hours just cutting a row of 5 units into the wall. You have to concentrate on the whole wall at once, 'cause the plaster all moves, you need to keep your wrist in the same position, you need to stroke at the right pressure, in the right place, at the right time, in a upwards direction, or it saggs. Leave the plastering, to a plasterer. It is a hard skill, you will deffently do a shit job the first time you do it... Why dont you just filler the scraper marks, then give it a couple of layers of paint. Or dot and dab with plasterboard may well be the easiest option. Deffently the least time consuming.