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In article ,
(Andy Hide) writes:
Many thanks for the useful advice. My practice session proved most
successful and I was pleased with the results. However, things have
gone downhill since then. I had another go last night and something
has gone very wrong. As I spread the plaster across the board I am
getting scratch marks. It appears that the plaster has gone gritty and
the small pieces of grit are leaving scratch marks as I move the float
across the board.

I've concluded this could be because:

1) Plaster not mixed properly. I tried adding more water but this did
not seem to help. I threw out one mix and started again. Same problem.
Bucket was fully washed out between mixes.

2) Plaster has gone off. The bag of plaster was left open inside the
house without being covered up. Would it go off in a day ? The house
is not damp.

Most likely you did not completely clean your tools.
When I mix plaster and pour it out of the bucket onto the
board, I then immediately go and clean the mixing bucket
and wisk, before even applying the plaster. It's easy at
that point, and the plaster can happily wait the extra
time.

Other possibilities are that your mixing technique left
some dry plaster around the top of the bucket, on the
wisk, or at the bottom which you didn't mix in. The
surface you plaster onto is unstable -- seal it with
PVA first (not applicable if you are still using
plasterboard). This might be from one of the surfaces
you are plastering up against, rather than the one you
are directly working on too. There's some other source
of contamination with dust. One amusing(?) one I've had
is plastering a wall which runs up to an artexed ceiling,
where the artex is in the form of small stalactites. I
found it very difficult to avoid breaking these of and
dragging them down the wall when working along the top
edge...

When you pour the plaster out of the bucket onto your
plastering table, work the plaster on the table a bit
with the trowel. This is useful to check you don't have
any lumps, but also any grit in the mix should come to
light at that point too.

--
Andrew Gabriel