I've had two plasterers in recently to bond and skim some walls. I'm finding it virtually impossible to measure the quality of their work - what do I look for? I've run a straight edge over the walls and there are 2-5mm gaps in places and they feel slightly wavy to touch. I've checked the levels against the skirting and cornices and here again, it's not exactly spot on. Is this common? if not, how do I rectify it?