Grumps wrote:
I need to lay a 3-4mm smoothing underlayment on top of a screed floor prior
to fixing vinyl tiles (Karndean). Any tips to ensure a smooth even surface?

Personally, I'd pay someone to do this job, who has done
it for a while before. The technique seems to be to mix
the stuff up and lay it on bloody fast, using a float
to distribute it quickly to an approximatly even
thickness, stopping after a few seconds, well before the
stuff's taken any sort of set.