hi there
I spent much of my formative years as a decorator and i came across this problem many many times.
Filling in knocks is crucial a top brand filler is required Fill in and leave to harden -several days at least Dont overfill too much.The darker colours should be okay the light colours can be tricky (Accept that you may have to coat the whole wall again) Touch up patch with brush - feathering out over a small area gently This can be done also with a roller over a larger area.
For small scratches or bare patches ,several coats of paint will be needed Alternatively you can also use water based undercoat for good results(trust me on this) and coat with wall colour once dry
richard pagan
January 2008