when i skim onto plasterboard i get a very reasonable finish and 1 im happy about .... but when i do a re skim on a normal wall i can never get the same type of finish as i do on the board . i more or less do the same things but it never turns out the same i,e a few blemishes or maybe a few slight ripples . any ideas as to why ? or is there something else i should be doing on a re skim????