great replies guys! THANKS.
i've had the same thing happen now on both overskims and on plasterboard.
Rich, this has only been happening recently, switched to a 2 mix system a while back so can safely rule that out... i think! was only feeling quite confident the other week and like you say, this came and bit me on the ar*e.
Mic, can't remember now where i've gotten all my muck from of late but certainly the last few bags have come from the same place and the keep it in a cold lock-up, thought this was ideal storage? as good as it would be to know that it was the muck and not me causing this, i kinda wish it was something i was doing, at least then i could fix it, getting bad gear i can't do much about, and yes it does seem greasy!! good description actually, been searching for a word to describe it.
Steve, well remembered! however i do now lay on quick and go back and flatten it in pretty well now so that a really thin top coat can go over it, so it's not that.
on reflection here's a more detailed description if can get it across:
i'll lay on and flatten off the first coat, goes ok and no strips, lumps or bumps.
then when i top it i'll wait around for it to feel firm enough to flatten, thing is it seems wet in areas but has pulled in along the trowel lines so i get on it and can't quite flatten the trowel lines out completely these then become my stripes, but more like slight bumps now, the rest of the muck on there is too wet still and get trowel lines a plenty when i've flattened in so am ending up troweling once more than i would normally now.
end result is that i finish with a hard trowel which kinda takes the tops off the stripes leaving it pretty good, fine when it's painted i'm sure of that but then looks kinda unsightly, and the troweling up is just not nice.