The fact you want to load up sand and cement with waterproofer is confirmation that you don't like suction. Any decent old school plasterer will tell you that suction is your friend and the key to quick high quality skimming.
Agree of course suction rate is key to producing a good finish in all aspects from skimming to rendering etc. Used to get caught out a lot when was younger either to much or not enough ( and recently with my hardwall f**k up) generally give things old walls, paper stripped walls another coat or in sand cement case another couple of gluggs of waterproofer as would rather wait then chase. Work quite a bit on me own so mindful of setting to quick while cleaning out knocking up etc. Having said this dry hardwall has always been grey area as conscious of pva skinning over so wet down first then pissy pva, but as said that kick me up the arse recently when went off on us an buggered few walls, which then hacked off an re did float an set as it was eating away at us