New shiny plaster: to sand or not to sand before painting?

[johnny2007]

This became a very heated argument yesterday between decorators working at my former customer new built.
Plaster is dried for 4 weeks and it's very smooth and shiny.
Some decorators want to give it a quick sanding before mist coat, others say it will ruin the perfect finish.
Got to say, I've never seen such a perfect plastering job, even in corners and difficult to reach places.
So over to a poll.
Disclaimer: I'm not a decorator, I don't like decorating and I leave it to the professionals, but I'm curious about this dilemma.
Different views even on this forum looking at previous threads.

 

[opps]

If you sand it with 180 grit on a random orbital (connected up to a dust extractor) you should not "damage" the surface. If you attack it with 60 or 80 grit paper on a sanding pole, then yeah, you will see score marks, particularly if you fail to dust off or vacuum the walls.

Overly polished plaster, can result in the mist coat not soaking in properly, which potentially means that the final finish coats will be prone to chipping.

I wonder if the decorators that object to sanding the walls would also object to sanding previously glossed surfaces.

 

[johnny2007]

2 votes, one for sanding, the other for not sanding.
Same as google and other forums?
Not enough votes to say...

 

[DiyNutJob]

If I am especially concerned about it, I would experiment. Sand a bit, and not sand a bit, and see the difference. If no difference, then no sanding for no work. My gut says the mist coat is to wash the plaster of dust to promote adhesion and to harden the plaster surface by activating any residual cementing chemicals. Therefore, no sanding is needed. Same style of activation using plain water also works for brick mortar.

 

[opps]

If I am especially concerned about it, I would experiment. Sand a bit, and not sand a bit, and see the difference. If no difference, then no sanding for no work. My gut says the mist coat is to wash the plaster of dust to promote adhesion and to harden the plaster surface by activating any residual cementing chemicals. Therefore, no sanding is needed. Same style of activation using plain water also works for brick mortar.

A mist coat is required to reduce the level of suction, it has nothing to do with dust, and gypsum plaster doesn't have cement.

If you put full fat emulsion over gypsum plaster, the water in the emulsion will be sucked in to the plaster before the molecules have a chance to cure via coalescence. Water based paints cure by letting the water evaporate off, as it does so the film shrinks and the molecules bond. If you paint full fat emulsion over new plaster, you risk the next rolled coat pulling off the previous coat, because the first coat didn't "soak" in to the plaster.

I have first hand experience of emulsion "sliding" on shiny plaster. The adhesion is almost as bad as painting emulsion on glass. Months later, you can scratch it off with your fingernail.

Ironically, on highly polished plaster, the emulsion cures correctly if used full fat (the coalescence process works as it should), but the paint does not adhere properly.

 

[DiyNutJob]

gypsum plaster doesn't have cement.

It has a bonding agent. I just call it cement, same way they call the UPVC pipe glue as cement even though it hasn't the same cement used in mortar.

 

[johnny2007]

I'm gonna run a test and see what happens

 

[opps]

It has a bonding agent. I just call it cement, same way they call the UPVC pipe glue as cement even though it hasn't the same cement used in mortar.

Fair enough but after a plastered wall has dried, there is no need to wet it to make it harder.

 

[DiyNutJob]

there is no need to wet it to make it harder.

But there is. If it's not harder, it will cause suction like you said. Hence, the mist coat is for the hardening and to bond all the surface dust/powder in place.

 

[opps]

But there is. If it's not harder, it will cause suction like you said. Hence, the mist coat is for the hardening and to bond all the surface dust/powder in place.

If that were the case, why not just spray a mist of water on the plaster and then use full fat paint once the water has dried?

We seem to be at odds here. I am not talking about the hardness of plaster when considering polished Vs over polished plaster. I am talking about adhesion and absorbency.

 

[DiyNutJob]

If that were the case, why not just spray a mist of water on the plaster and then use full fat paint once the water has dried?

You could, but it's less effective than using a mist coat. The bonding agent in the paint is needed to bond the surface powder in place. The paint also allows you to see which part is done and which part isn't. I have just made all this up. So you don't have to believe me. But, I am so convincing, I even believe me myself!

We seem to be at odds here. I am not talking about the hardness of plaster when considering polished Vs over polished plaster. I am talking about adhesion and absorbency.

Surface debris (I expect it to be gypsum powder) will interfere with paint adhesion. Water absorbency will also interfere with paint adhesion. Mist coating deals with both. I deduce surface hardening will likely happen from my observation of cement mortar, where additional water is needed to activate the bonding chemicals on the exposed surface. Same process will likely happen for other kind of materials where bonding agents are involved.

 

[johnny2007]

Well, the decorating is starting on Monday, so I've just been there and done a test.
One bit sanded with 180 grit paper.
One bit using a 3m scratch pad (the grey one, less abrasive.
One bit left as it is.
All painted with a mist coat (25% water, 75% dulux emulsion as per dulux instructions, measured with a large syringe so to avoid excuses).
I'll go there tomorrow or Sunday and try to scratch it off with my fingernails and/or a scraper.
Let's see what happens

 

[johnny2007]

Ok, just been there and scratched the paint with my fingernails.
These are the results:
1. Unsanded plaster: the paint can be scratched off without much effort, exposing the plaster underneath.
2. 180 grit sanded plaster: to scratch the paint, i needed to apply some good effort and plaster becomes exposed after a couple of attempts.
3. 3m scratch pad sanded plaster: this seems to be the best method of the 3. It is difficult to scratch the paint and after some effort, only the superficial paint can be scratched off, but the plaster doesn't get exposed.

So, it seems that a light rub gives best results allowing the mist coat to penetrate the plaster properly.
This is after only half a day drying time of the paint.
I will give it another go tomorrow morning if I can; maybe the paint needs to cure a bit more.