In article , "Pecanfan"
says...
snip

Also, for future reference, Thistle Bonding Coat should NOT be left to dry
out before applying subsequent coats (D'OH!!!). It's so ultra-absorbent the
skim dries the second it touches the basecoat and a) makes it completely
unworkable and b) stops the skim from sticking properly. I found this out
the hard way when a large section of skim 'fell off' the wall. IF the
basecoat dries out you MUST re-PVA the wall before applying the final skim.

You can just dampen the bonding with a water spray, or use a really
sloppy mix for skimming.