Must agree with sandtex but there is a company call BEDEC they do some awesome stuff realy flexible gear. I've used ALOT on new build renders that have cracked and crazed after a few years. Top stuff
If your leaving lines on the wall then your not back rolling enough, ie bad technique.
Start a full roller in a big 'W' shape then keep rollering over that area filling in the spaces until the coverage is even. look on you tube there MUST be a video!
Best bit of advice is to sign up with a few agencies. There you can work alongside more experienced decs and there's no time pressusre on you. Agency workers are always dead slow!
And ignore people who say 'if you can **** you can paint' You've got to be spot on to earn a living at it!
Use them for 'cutting in' the celing line if your a novice. but a roller is definatly the way for the main area's. Alot of the dec centres sell pacages of 9" sleeve frame & scuttle for cheap. Better quality than the b&poo stuff!