Plaster Skim Tips

I have been practicing skimming onto some scrap pieces of plaster board which I have taped together and temporarily fixed to a wall.
I'm using Thistle Multi-Finish and was hoping for a few tips.
1) One coat or two ? From other postings in this group it would seem that you should apply two coats in fairly quick succession. This will increase the time it takes to dry out and give you more working time.
2) How long to wait between applying the second coat and starting to polish ? Does this just come down to experience ? I probably waited about 1/2 hour.
3) When you are polishing should you be using the trowel flat against the wall or at an angle ?
4) During the polishing I seem to keep ending up with a small amount of sludge on the trowel which constantly needs to be cleaned off. Is this normal ? I am keeping the trowel wet and using an old paint brush to dampen the plaster on the wall before polishing ?
5) Does the plaster change colour once it's polished ? Seemed to turn a bit darker after a few passes of the trowel.
Any help is much appreciated. The ultimate aim of this "practice" is to skim a plasterboarded ceiling. Is this much more difficult than working on walls ?
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Best tip is to use Gypsum Easy-Fill for the final coat. About five times more expensive in B&Q (10/10kg) but a hundred times easier to get a good result. Also it is a lot stickier so you plaster the ceiling, not yourself.

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     snipped-for-privacy@totalise.co.uk (Andy Hide) writes:

I usually apply 2 coats, but I'm rarely working on a surface as good as new plasterboard. I have done just one coat when the underlying surface was good, and that one coat went on well.

That's not the reason (and I don't think it's actually true either). When you do 2 coats onto rougher surfaces such as a scratch coat, the first coat goes on and pretty much fills in the surface bringing it all up to the level of the highest peaks on the surface. You couldn't polish this because in places it will be level with the peaks of the scratch coat. The second coat then goes on to make sure there's a layer of finish plaster over the top of all the highest peaks in the scratch coat, and this can be polished without the scratch coat peaks showing through.

You start by just getting the plaster on the wall, as evenly as possible, but ignore the trowel marks which you can't do anything about at this stage. When the plaster has gone off a little and got a bit thicker, you will be able to re-trowel the surface and produce smaller trowel marks -- if you can't, then leave it a bit longer. At each stage in the 'going off' process, you will be able to improve the surface a bit, but no more. An important part of the process is not to carry on trying to improve the surface past what is possible at any particular point in the 'going off' process -- all that can do is make it worse. You must leave it to 'go off' some more before you can improve on it any more. Constantly over working an area in an effort to try and improve it is a very common reason for poor outcome.
It's impossible to give timings -- depends on temperature, age of plaster (how long in storage), how you mixed it, how long before it went on the wall, absorbancy of the wall, phase of the moon, etc.

Always at an angle. If you accidently get the trowel flat on the wall, you'll be very lucky if you get it off without yanking off a trowel shaped chunk of plaster.

Yes -- actually it's extremely valuable for filling in blemishes. Polishing is all about scraping off the minute peaks and dropping them into the minute troughs in the surface, so you don't want to be wiping too much off the trowel -- it's part of the polishing process.

That's fine. A misting spray can be useful too, although a professional wouldn't be seen dead with one -- accurately aiming the water from a distemper brush is more professional;-)

No -- the change in colour is the plaster going off. It will happen regardless of you polishing it -- it's a chemical reaction. The plaster goes back to the lighter colour when it dries out (which is nothing to do with hardening, except it mustn't dry out until it has fully gone off).

The hard part is stepping back to see how you're doing...
--
Andrew Gabriel

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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.
I am determined to master this!
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     snipped-for-privacy@totalise.co.uk (Andy Hide) writes:

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

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