plastering

A Question 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 people 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 coat. 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 the float. 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. Regards Joe
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     snipped-for-privacy@DIT.IE (JOE) writes:

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.
--
Andrew Gabriel

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ANDREW THANKS FOR YOUR SUGGESTIONS ON THIS PROJECT REGARDS Joe
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