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?
I thought either;
1) Just mix it up and pour it on; it's really easy; just do it! or
2) Use an 8mm notched adhesive spreader thing. This would ensure that the
underlayment is applied to the correct thickness, then I could smooth over
any ridges that didn't self-level.

It's going to be about 100kg of mix, and it'll be mixed in several loads. Is
there anything I should be aware of when applying a new load to the edge of
the existing stuff?

Thanks all.