Painting new plaster - how long to wait?

[topt]

Replacing ground floor full height L shaped windows with AAC blocks which will then be rendered/plastered. Plaster probably at least 15mm thickness, possibly 20mm.

 

How long should one wait before painting especially considering current humidity?

 

Can you also recommend best practice here - any specific sealer to use or use diluted emulsion - I believe it is now called a "mist coat"....... 

[sometimewoodworker]

11 minutes ago, topt said:

How long should one wait before painting especially considering current humidity?

 

Can you also recommend best practice here - any specific sealer to use or use diluted emulsion - I believe it is now called a "mist coat".....

You can paint as soon as the render is dry (2 to 3 days) just make sure that you get paint designed for new render, just ask any paint shop, you can thin the first undercoat by about 20%

[lemonjelly]

To seal plaster and help paint adhere: Mix 1 part PVA wood glue to 4 parts water and brush/roll it on. Let it dry before painting over it. 

[Crossy]

3 hours ago, lemonjelly said:

To seal plaster and help paint adhere: Mix 1 part PVA wood glue to 4 parts water and brush/roll it on. Let it dry before painting over it. 

 

I wouldn't do this with new render, the "seal" will potentially trap the remaining moisture in the render.

 

As STWW suggests, get the proper stuff and you're good to go.

[topt]

8 hours ago, sometimewoodworker said:

You can paint as soon as the render is dry (2 to 3 days)

Thanks this is what I had thought until looking it up on the net and some sites (UK/US/Aus) were talking 3-4 weeks to let it dry out properly for internal walls..........

[sometimewoodworker]

1 hour ago, topt said:

Thanks this is what I had thought until looking it up on the net and some sites (UK/US/Aus) were talking 3-4 weeks to let it dry out properly for internal walls..........

There are paints available for old cement, 30 days plus, and paints for new cement, 2~3 days, they are not the same. 
 

It is nothing to do with drying time (you DO NOT paint wet cement) it is the alkalinity of the surface and substrate you are painting. All interior (and most exterior) paints allow water vapour out and prevent liquid water getting in (bathroom paint may be an exception).

 

It’s quite possible that there may be an advantage in some way to using paint that is not alkali resistant, if you want to know you will need to a paint material scientist at one of the big paint makers and hope you gets truthful answer.

[canopy]

Note your render is quite a bit thicker than what you want which is 5mm so you'll be buying 3 or 4 times more render and since render is a thermal mass overdoing it is not good for keeping a house cool. The first 3 days after the render goes up should be spent methodically watering the render minimum 3 times a day to cure the cement to an appropriately strong level and prevent cracking. Once cement dries it stops curing and the process can never be restarted again.

 

And I haven't seen the word primer appear yet. Before you think about paint, you need to primer the walls. The primer you choose will often say how long to wait after rendering before applying. Often 30 days of drying time is desired, but some primers, like expensive oil based ones, claim it can be down to a matter of days.

 

[topt]

50 minutes ago, canopy said:

Note your render is quite a bit thicker than what you want which is 5mm

yes I got that bit wrong - guy is doing the blocks now and the render won't be any where near the thickness I mentioned fortunately.

 

[canopy]

Good. The render thickness is often a function of the accuracy of the block laying. The ideal 5mm is more theoretical than practical. Since the minimum render thickness should never be less than 3mm anywhere that means in order for a wall to be suitable for 5mm thick render no block can deviate over 2mm anywhere across the entire wall. That's hard to do. I builder once told me about a wall built so sloppily that he had to use 12cm thick render--thicker than the blocks themselves! It pays to be accurate like using lasers and string lines 

 

[sometimewoodworker]

2 hours ago, canopy said:

And I haven't seen the word primer appear yet. Before you think about paint, you need to primer the walls. The primer you choose will often say how long to wait after rendering before applying. Often 30 days of drying time is desired, but some primers, like expensive oil based ones, claim it can be down to a matter of days.

I was using the word paint assuming that it was generally known that painting involves 1 to 2 coats of an undercoat  followed by 1 to 2 coats of a topcoat.
 

As I mentioned every paint shop has undercoat that is specifically OK after 3 days it’s available in water based acrylic as well as an oil based one. Virtually all of our walls have been painted with one of those water based ones and we have had no pain failure on any wall.

 

The render will be dry in 3 days, it will be mostly cured in 30. 

[canopy]

16 hours ago, sometimewoodworker said:

The render will be dry in 3 days

Which is why good builders keep the render hosed down and wet for the first 3 days. It is best to follow the simple instructions on the bag that are intended to assure you make a top quality job.

 

[petermik]

Try this....good stuff but not cheap.

Edited April 3, 2021 by petermik

[topt]

3 hours ago, canopy said:

Which is why good builders keep the render hosed down and wet for the first 3 days.

I saw a guy doing that for the columns of a build behind me 10 years ago and for longer than 3 days but not on the walls.

Everybody seems focused on the external wall........or are you suggesting I do this for the internal rendering as well ????

 

[kamalabob2]

The Op might consider "Skim coat" if he wants a smoother wall finish.  Sure there is both oil base and/or water base "Quick primer" from all the major paint brands. I am not sure I would primer after waiting only 3 days. The buckets of primer are often in both English and Thai languages. 

[canopy]

20 hours ago, topt said:

are you suggesting I do this for the internal rendering as well ????

Absolutely yes. The most common reason render cracks is improper curing. You will see builders methodically going around the inside and outside of buildings with a garden hose to keep it good and wet for the first 3 days after rendering. Not only that, they also know to wet the block walls before applying the render. It's not my suggestion, but rather it's about understanding how cement works and following proven industry practices instead of just slopping it on with no knowledge and not bothering to read the package instructions.

 

[Yellowtail]

2 hours ago, canopy said:

Absolutely yes. The most common reason render cracks is improper curing. You will see builders methodically going around the inside and outside of buildings with a garden hose to keep it good and wet for the first 3 days after rendering. Not only that, they also know to wet the block walls before applying the render. It's not my suggestion, but rather it's about understanding how cement works and following proven industry practices instead of just slopping it on with no knowledge and not bothering to read the package instructions.

 

 

I think a pump sprayer is much easier to use than a hose-pipe...

[canopy]

1 hour ago, Yellowtail said:

I think a pump sprayer is much easier to use than a hose-pipe...

Sure, and a lot less mess too.