What causes this? Pimples?

[loply]

Hi folks,

Been doing a fair bit of plastering lately and I'm getting quicker and better.

Occasionally I have a problem when I get the first trowel on with this kind of spotty pattern appearing. They're like little volcanoes on the plaster. I can usually get rid of them eventually but they're a nuisance.

What causes this? Is this what people call pimples?

This was a reskim on an existing plastered wall if it's relevant, PVA'ed 4:1 the day before then again 20 minutes beforehand.

Cheers,
Rich

 

[kjacko]

I could be wrong, and i've never really had that issue myself as a fairly competent diy plasterer, but someone recently told me bubbles like that were due to not allowing the mix to settle before applying it to the wall.

Personally, i've always left it about 10 mins, then banged the bucket a few times to release any trapped air to the top.

I've seen our site guys at work do this with easyfil. If they don't they get the same bubbling effect.

 

[loply]

I do notice lots of bubbles / air holes when I first spread the plaster onto the wall. I usually then pull back over the mix with the trowel in the opposite direction to "blend them out".

In this instance however I did leave the mix sitting for a while (5-10 mins) as the PVA wasn't ready yet.

 

[ree]

It could be the mix - too much water or not hand mixing it thoroughly (energetically). If you are using a power paddle then its easy to overdo it and introduce air.

Or the last coat of PVA was washy and has introduced too much moisture.

Allowing tile grout to sit and slake is conventional but i've never done it with float or skim plaster mixes.

Thing is, with this kind of post its difficult to be definitive without seeing the backing.

Maybe someone with more experience than me will come along?

 

[NickB_99]

What causes this? Is this what people call pimples?

I've seen posts on this before. They've been described as blisters too. Worth a search.
Though from what I recall, continuing to trowel over them can cause more to appear.

Not sure exact cause, but I've had them occasionally when re skimming old walls.
If one or two appear, I go easy on troweling that area and when dry give a quick run with sanding block and fine sandpaper. Prob not the ideal as plaster shouldn't be sanded, but seems to knock them off ok.

 

[roy c]

If you get them you can brush them out and leave it for a while till the plaster "firms" up a bit.. Try not to use sand paper or sand them later ,,It's bad for our image (Only kiddin'Newby ),,,

 

[noseall]

Have a look here.

 

[loply]

Thanks folks, few things to try next time there.

I reckon I probably wet the wall too much with the PVA and/or got on the trowel too early.

 

[Joet]

You are correct, it's from moisture on the surface. Nothing to worry about and will flatten out.

 

[joe-90]

Nothing to do with moisture, It's tiny pockets of air on the surface causing it.