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In article ,
(Andy Hide) writes:
Thanks for these tips. Another question though. If you are skimming a
largish wall e.g. a hallway or stairs is it possible to split the job
into two sections ?

Yes. Pick a join line, plaster slightly over it, and before the
plaster has gone fully off, cut back the edge to a clean step, and
not feathered, as you can't feather in a join. If you know where the
join is, you will always be able to see it on the finished work, but
a good join won't notice unless you look for it. On many houses, you
will spot a join on the wall where the staircase is, as the plasterer
probably didn't do it in one go.

Skim dries out quite quickly and I don't think I
could get a 4m wall done in time.

Hum -- skim normally takes a day to dry out IME, much longer than it
takes to go off (set).

Anyone have any tips for skimming onto old plaster? Managed an almost
perfect skim onto plasterboard but finding that old walls are much
more difficult. Any slight lumps and bumps mean that the you are more
likely to miss bits when trying to get a good finish.

Remove any obvious lumps above the surface.
The first skim coat goes on normally as thin as you can but should
leave the plaster level with the most proud bumps remaining on the wall.
i.e. you tend to scrape the trowl edge over the rough surface, but it
should leave you with a flat surface. The reason for the second coat is
that you can't polish this first coat because of the background coming
to the surface in places. So the second coat is then put on ~2mm thick
to give you clearance over the most proud bumps on the background, and
that's enough to enable you to polish the surface.

I primed the surface first with PVA which I think helped to increase
the working time. Total thickness of the skim was about 2-3mm. Can you
go thicker than this on old walls to help cover up any imperfections ?

Yes. The issue is that plaster finish coat shrinks when it sets. A thin
coat shrinks by getting slightly thinner which doesn't notice, but a
thick coat may crack.

Applied in two coats, the second applied just as the first was
begining to set.

Also found that there were a few grit marks. Badly mixed plaster ?

Possibly. Check you aren't leaving unmixed plaster up the sides of
the bucket, only to fall in when you pour the plaster out. Other
sources of grit are from the wall (PVA should have stopped that)
or from adjacent walls/ceilings. I had fun plastering a wall up to
a ceiling artexed with small stalactites. The trowl edge would keep
breaking them off and dragging them into my plasterwork ;-)

When ironing out the trowel marks as the plaster is setting does
anyone have any tips on which way to work across the wall. Should you
go top to bottom or left to right? Are long sweeps, short sweeps
better ?

Generally inwards from all the edges. Keeping the edge of the trowl
parallel to the wall edge, start the sweep along the edge but in a
circular motion which sweeps out and ends a quarter turn later in a
direction perpendicular to the edge, but without turning the trowl.
When you aren't near an edge, it doesn't matter.

--
Andrew Gabriel