Air bubbles in plaster

[john59]

Cmon legs akimbo and other very talented plasterers, I have just plastered a chimney breast using multi purpose skim over browning and when I was troweling up I noticed small bubbles (about the size of 5p pieces dotted about , these seemed to be air bubbles, what did I do wrong and how can I avoid them in he future.
Many thanks in advance for your great wealth of knowledge on this very noble art.

 

[legs-akimbo]

Hello John
My guess is you either skimmed up too early on the browning, as in before it had time to set, or perhaps too late and it was thorougly wet down before you skimmed up. Either of theese are the most common causes of "Blebbing" which are small pockets of air trapped behind the skim. They usually trowell out right at the end of the setting of the plaster, or if you can not get rid of all of them like this then wait till the plaster has set and come back to it after a couple of hours and with pressure on the trowell give it a good dry trowelling, this should do the trick.

 

[olly_k]

hope you don't mind me assking but what do you mean by 'thoroughly wet down by the time you skimmed up' - I also seem to have probs not with browning but final skim This is one area I would get the pros in every time if I could afford it!

 

[jbonding]

its all about when you turn back on it after floating, skim it too soon or wet too much may cause blisters.it comes with experience, but happens to experienced plasterers, but dont panic like legs says get them out just before it sets.

 

[john59]

Cheers Legs, I think your first explanation was correct , I had floated the wall with the browning but it was taking ages to go off I was in a hurry to get the job done so I decided to skim up while the surface was crusty to the touch but when I pressed on the stuff it left an indentation. I will not do that again. By the way ,any thoughts on why the browning took so long to go off? at its thickest point it was about 1 inch deep.
Incidently I plastered up my first room today just skimming over existing plaster that had been knocked about a bit when removing woodchip wallpaper. There are two walls 12 foot by 8 foot tall and two walls 10 by 8 with a window and door , using multi skim I knocked up a buket of plaster and started to spread it on the wall, no sooner had I finnished the bottom of the wall and started on the top section the plaster had started to go stiff, I used clean water and everthing was spotless and I was chasing it all day like that and boy my arm is falling off. Got it done though so well pleased with myself

 

[bikerm4]

Cheers Legs, I think your first explanation was correct , I had floated the wall with the browning but it was taking ages to go off I was in a hurry to get the job done so I decided to skim up while the surface was crusty to the touch but when I pressed on the stuff it left an indentation. I will not do that again. By the way ,any thoughts on why the browning took so long to go off? at its thickest point it was about 1 inch deep.
Incidently I plastered up my first room today just skimming over existing plaster that had been knocked about a bit when removing woodchip wallpaper. There are two walls 12 foot by 8 foot tall and two walls 10 by 8 with a window and door , using multi skim I knocked up a buket of plaster and started to spread it on the wall, no sooner had I finnished the bottom of the wall and started on the top section the plaster had started to go stiff, I used clean water and everthing was spotless and I was chasing it all day like that and boy my arm is falling off. Got it done though so well pleased with myself

Did you pva the wall first before overskimmimg?

 

[jbonding]

alright john, the reason plaster doesnt set quick depends on the suction of the background or how the background was prepared. I was grafting today, not plastering but boarding and if the weather was as warm as it is was where i am, thats the problem. the only solution in warm weather is shut the windows,mix it wetter and dont put on as much as you normally do.

 

[legs-akimbo]

If you only have small manageable areas to brown and want to get skimming up an hour later then add in a trowell full of hydrated lime to the browning for a "flash set" this will give you time to get it on and rule it straight but will set much faster.

 

[john59]

Thanks chaps, I remember back in the early eighties when I was labouring for a plasterer I used to always add lime to the mix when doing internal rendering in new build houses but I didn't know why at the time (this was using sand and cement) now I do . Incidently when I was doing the job where I was chasing the plaster before it set last week I did PVA the walls first but it seemed to make no difference as the plaster was setting quickly on the spot board could this be due to the age of the plaster I bought? I did use clean water for all the mixes and cleaned the bucket out each time. Just to add I am plastering up two walls this week which have had radiators removed and I have put matches in the holes where the brackets screw on , good idea or is there an alternative?.
Rgards.
John.

 

[Porker]

Old plaster will go off quickly. It is date stamped on the bag and I believe the life is around 3 - 6 months. Personally, I think its never worth using old plaster just bin it. Also the warm weather doesn't help. Worth keeping the windows shut and splashing a bit of water around to up the humidity.

 

[jbonding]

might as well leave the screws in, remember its behind a rad so i wouldnt worry to much.