I am about to patch an area 6 by 4ft on an existing plastered wall.
I have searched the site for advice on the subject and like most
the idea of plastering and matching onto an old plastered surface
(block wall) is going to be a difficult task (there is going to be
some lighting in the area so any bad plastering will show up).But i am
going to have a go at it. The suggestions given on the site are very
useful. The problem I expect to have is (a) Matching the old with the
new (feathering)and (b) Obtaining a good flat finish on the final
With regard to (b) I hope to level off the base coat with a piece of
wood, with two vertically lines of partially set browning, which I
dont expect to be a major problem ,The problem I see is putting on the
final coat, how does one get it level and flat while its drying fast?.
Has anyone used a roller to put on the finishing coat?. With regard to
(a) the suggestion I got was to use of a hard wet sponge combined with
Has anyone any suggestions on this topic
I believe there will be a new product from Belgium on the market in
the near future which will make the plastering of a wall much easier ,
I saw a plasterer/ painter using it on a television programme he
seemed to use what looked like a tool like a T piece with some flat
rubber on the top of the T piece.The plaster was placed on this flat
rubber and was rubbed onto the dry lining surface.
Has anyone seen or used this product.
You can't feather plaster, as you can't apply and polish an infinately
thin layer. What you could do is this... Use a stanly knife (new blade,
but you'll chuck it out afterwards) to cut down the surface of the
existing plaster some 4 inches back from the edge where you will be
joining. You should be able to remove the existing finish coat from
this 4" strip (doesn't matter if you go deeper in some places, but
make sure all the surface is removed). This strip now forms one of
your battens for the scratch coat. You will need another one at the
other side of the hole. It's not clear to me if you are running
into a corner, or if you have plasterwork all round the whole, but
you can use a wooden batten at the other side if there isn't plaster
to form the other batten. If you are running into a corner, space that
batten some 12" out from the cornet. If the scratch coat plaster you exposed is
not flat, you can add a batten level with it. Pack the battens behind
so they are vertical and don't move when you press on them (or so they
are evenly 3mm behind the finished surface if you are fitting one near
the join). You can use a taught string spaced 3mm in front of a batten
distant from the join and extending past the join by many feet to
ensure the wall will be flat over the join (string should not be bent
by the existing plaster at the join, nor spaced away from it).
Now you have your two battens for the scratch coat surface. You will
need a straight edge (piece of stright timber) to stradle the two
battens with at least some 4" of overlap. If this is excessively long
(more than 5'), use more battens across the hole.
You will need to prepare the blocks for plastering. If these are
thermal blocks, be careful not to get them wet through, as they will
expand, which might damage the wall, and they will shrink when they
dry, cracking your plaster (and maybe making it fall off). Wetting
the surface with a brush is fine, but don't spray them with a hose!
Paint the surface with dilute PVA, and when that's dried, repaint.
This will seal the surface and stop it instantly sucking all the
moisture out of your plaster before it sets. Then apply your scratch
coat. You apply it to be slightly proud of your battens. Then,
starting from the bottom and working up, use a horizontal sawing
motion with the wetted straight edge against the battens to 'saw'
off the plaster which is proud of the surface. Stop and wipe the
plaster from the straight edge when it builds up. Inspect the area
you ruled off to make sure it all shows the horizontal sawing marks
from the straight edge -- any which doesn't is behind the level of
the battens and you should apply more to that area and repeat.
When the scratch coat has gone off (doesn't need to be completely
hard), apply the finish coat. The first coat should be as thin as
possible and basically fills in all the surface blemishes of the
scratch coat, but thick enough that no bumps from the scratch coat
show through. However, first just get it all on the wall -- until
it starts going off a bit, you can't do anything else with it.
When it does start going off, you can start going over the trowel
marks and the new ones you create will be smaller. When it's gone
off some more, you can go over these and eventually you will do
this without creating new trowel marks. Then you repeat with a
second finish coat, and that should butt-join with the existing
plaster where you cut it with a stanley knife.
Actually you mean 'set' or 'go off', which is nothing to do with
drying -- it must stay damp until it has completely set or it
will stop setting. It doesn't set fast -- you have ages to work
on it. As I said above, you start just by getting it all on
the wall. Then you have to wait for it to go off a bit and you
can smooth out the trowel marks, and you repeat this as it goes
off more and more. Don't keep working on one part though -- when
you've gone over it, you have to wait for it to go off some more
before you can improve on it. When it has started going off, use
water (misting spray is easiest) to lubricate the plaster so the
trowel edge glides over it. The water carries the 'bumps' of
plaster which the trowel edge removes, and deposits them in the
holes. This is the basis of polishing. As the plaster goes off,
you will be polishing at a finer and finer level, and pressing
harder on the trowel.
Final comment; practice somewhere first, like in the cupboard
under the stairs, or inside the garage.
If you do, please get someone to film it, and post here when it's
due to be shown on "You've been Framed" ;-)
Yes, feathering won't work.
New houses are moving away from using wet trades as fast as they
can as it's an on-site skilled job, it's too expensive to factor
that in, and increasingly difficult to find the skills. Ceilings
mostly haven't been plastered for more than 10 years now, and
increasingly, walls are often just plasterboard.
A friend near St.Albans who is getting quotes for an extension has
been told by builders that they can't get plumbers at all now, and
electricians were becoming difficult too, apparently because of a
number of large projects like Heathrow T5 grabbing all available
ones. A year ago when he started talking to builders about building
this, they were not at all happy when he suggested he would do
electrics and plumbing, but now it's the opposite, as otherwise
they can't do the building job at all.
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