Plastering - rough spots and lines

Hi,
I've read many great guides on this newsgroup on plastering and so I thought I'd have a go at replastering some small areas of a bedroom.
Mostly it went well (I got more on the walls than on the floor!) but I did get some rough spots which did not polish out. They are small but look "gritty".
The worse problem is that, although the plaster looks smooth at a casual glance, it is actually slightly "ribbed". The old plaster is also like this but I don't think this looks ok.
Does anyone have any tips for preventing these problems in the future?
TIA, Mark
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Mark wrote:

Practise. The rough spots probably never got trowelled properly - are they slightly lower than the surrounding work ?
If you look at the job with a light at a shallow angle you will be scared silly. Even on pro work !
Fill the rough spots with a fine surface filler. Sometimes the ridges are almost just water lines, and can be sanded out. However, if the whole surface is zig-zagged in and out of the wall, you have been putting more pressure on one side of the trowel than the other. There's not much to fix that except re-skimming. Simon.
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Mark wrote:

What were you (patch?) plastering on? Lime plaster - render - gypsum plaster - ?

Contamination of the plaster with something from the base?

Rbbed, or undulating? If ribbed, it's from your float rattling on the surface. More water, less speed, go at it from another angle.

Do it again (and again..).
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wrote:

It's a mixture of new render, old blocks, and gypsum plaster. (I'm using thistle multi-finish.)

I don't think so. I was very careful to remove all loose material before starting. I used two coats of diluted PVA before plastering. (the first allowed to dry, the second immediately before plastering).

Ribbed. I did use quite a bit of water - it was dripping off the wall in places.

Thanks ;-)
Mark
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Mark wrote:

FWIW letting PVA dry before plastering (or rendering, or repairing concrete) isn't really a great idea, it can make plastering difficult, and the PVA doesn't work well. Just swish it on and go straight onto it.
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IME, just the opposite, I've only done bare brick (super dry, super suck) & 2 coat browning/multi, I always do 2 coats of pva and alow to dry thoroughly. Works fine.
-- Mike W
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VisionSet wrote:

Browning/multi should be skimmed the same day and it doesn't need any pva.
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It does in my house (super suck). Perhaps I should use the one for high suck backgrounds? But brick isn't supposed to be that suckee.
-- Mike W
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VisionSet wrote:

What i mean is that the browning should be applied (either with or without PVA'ing the bricks) and then skimmed the same day, preferably within a few hours.
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Yes I agree, this is what I do.
-- Mike W
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VisionSet wrote:

Perhaps it's built of "selected regrades". These are super-absorbent and need dunking even to lay 'em.
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VisionSet wrote:

It doesn't work as intended (unless you simply want to reduce suction, and aren't interested in any bond). You can also get extreme problems trying to plaster onto an impermeable substance.
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I've had to hack it off, chasing etc. It is bonded good and proper! I guess it isn't actually forming a plastic coat, rather it is sucked in far enough to leave a degree of suction.
-- Mike W
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Chris Bacon wrote:

I see,so if you have a wall 10'x9' YOU can actually cover that wall before all the PVA has dried out? bearing in mind PVA on a wall gets sucked in and dry in seconds.
--
Sir Benjamin Middlethwaite



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The3rd Earl Of Derby wrote:

Not if you use a roller on a brush steel, and it doesn't matter about it soaking in, it's already killed the suction by then.
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The3rd Earl Of Derby wrote:

a) the PVA should be dilute, and doesn't actually dry hard in that time;
b) You should only apply PVA if needed - it often isn't.
c) Your oppo. can help. I nearly said "it takes two to t---o", but thought it might attract dribble.
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If the ribs are the order of 1 - 5 cm apart and generally horizontal then this is likely to be too wet a mix, I think its called sloughing.
-- Mike W
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Mark wrote:

Once the plaster is on, you've got to keep it wet and troweled until it looks something like smooth, allowing it to dry out/partially set before trying to trowel up is a mistake. The surface to be plastered should be moist or sealed with PVA - trying to skim over bone dry backing plaster is a recipe for disaster, and you will almost always end up with a finish that resembles a crocodile's back. (seriously, it's like lots of squareish segments)
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wrote:

I only applied water when polishing using a mist sprayer. Do I need to apply water before this? The plaster looked very wet at this stage.

I did forget that once and the plaster looked like a dried up river bed.
Mark
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Mark wrote:

The ridges are caused by the trowel vibrating over the surface,you should be able to hear and feel this action when polishing. If so immediatly smoothout this area.
--
Sir Benjamin Middlethwaite



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