Bubbles in my plaster - any advice??

29 Jan 2008
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United Kingdom
Hi All,

I’m a diy plasterer, a relative novice as well – I’ve done about 12 walls of varying sizes now and 1 ceiling (never again!). The later walls have turned out ok. I would completely agree that it’s all about practice practice, practice!!

I’m at a stage now where I’m trying to reduce the amount of remedial filling and sanding that I have to do once the plaster has dried I would really welcome some help with a problem I’m coming across.

On the last few walls I’ve done (2 onto existing plaster, 1 onto plasterboard, always using multi) I’m finding a lot of bubbles in the plaster when I’m applying it to the walls. Well, initially on the first coat there's just loads holes in the plaster that I’ve applied (where seemingly air is in the mix and the bubbles have popped).

Once the second coat goes on I can often see small bubbles forming below the surface of the plaster, which generally need to be popped with the corner of the trowel because they won’t smooth. Unfortunately there’s been too many bubbles to get them all, therefore causing me much more sanding/filling at a later date.

Some my question is, how do I stop these bubbles??

I always follow the prep-rules when preparing to plaster and use the appropriate PVA/water mixes. However, as my bubbles appear to be on old plaster and plasterboard backgrounds I assume it’s not the prep/background causing the issue.

I have recently invested in a paddle mixer which does save me no end of time. At the risk of being a bad workman who’s blaming his tools - does the mixer speed (usually I have it on max) have any effect on the amount of air being brought into the plaster mix? Should I ensure the paddle always stays submerged at the base of the plastering bucket?

When learning to plaster (yes, by DVD) I was shown to just apply the first coat, leave to go off, then apply the second coat and do the dry/wet/wet/dry smoothing stages. However, as my trowel skills aren’t great I generally give the first coat a dry trowel after application to smooth it a bit (and to remove the holes caused by the bubbles) before leaving it to go off ready for the second coat. Could this be my problem – should I just leave the first coat well alone?

Apologies for the length of the post, and thanks for sticking with me this far! I though it was best to explain in full what I’m doing/doing wrong in order to receive more appropriate advice.

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you can have air in the mix which will cause very small holes or creaters when popped and are usually filled with the 'fat'. you can actually hear them popping when skimming a high sucvtion wall!

these often occur with machine paddled muck and are very little trouble.

then there are blisters. :evil:

these often occur when you try and skim backing coat (bonding or browning) before the backing has had chance to fully hydrate or with excessive pre-skim wetting. it can often occur when the substrate masonry is particularly non-porous.

you have to grind these out with excessive trowelling as the skim is setting.
I go along with Noseall, better to trowel away the blisters and bubbles rather than sand them down. You ruin the texture on the plaster surface with sanding. I know plaster can sometimes take for ever to go off, and the more you trowel over them, the more of them appear, only bigger. Ease off with the water, be patient, and trowel them down when the plaster begins to set.

It’s difficult to say what you may be doing wrong without actually watching you. You may be over aerating the mix; keep the paddle speed at moderate to low & use a circular motion, around the tub, rather than up & down as this tends to draw in more air. The mix may be slightly too thick & this tends to trap air & makes it more difficult to dispel as you float on the first coat; it also makes it over thick! Your trowel action (both angle & speed) could also be a contributory factor. You’ve obviously mastered the basics; try practicing different techniques on a gash bit of board (or wall & scrape it off before it sets) to see how you can improve your technique.

I get the first coat on as quick as possible & work it reasonably flat as I go, around 4 sq/m at a time, & then leave well alone until applying the second coat, which I always keep nice & thin but still workable without falling all over the floor! If I find bubbling is a problem, again, I get it flat & then leave well alone until it’s almost set, over towelling at this stage will only make things worse & you will end up with ridges. I then do as noseall suggests for blisters & use a heavy trowel action with the minimum of water when it’s almost set; this tends to leave a marbling effect & it can go a bit shiny but at least it will be flat. Whatever you do, don’t over wet it or the surface end up with loads of hollows full of powder.
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Thanks for your time and your comments – all really useful!

Sounds to me like it’s a combination of things I need to look at. I wasn’t aware that too much water could be a bad thing, so had been happily slapping it onto my walls but the bucket load during my plastering! I’ll be a bit more frugal with my water from now on.

Thankfully my new house is a bit of a state, so I have plenty of walls to practice on :D
Sounds to me like your mix is too whipped and you are applying it too thin. Slow the mixing and apply it a bit thicker.
I'd say over trowling as well. First coat on, leave it dont mess let it stiffen, second coat on trowel up a little leave alone untill its ready to hit it.

If you try to atchieve a flat smooth finish to early, you will get Blisters as noseall says. I'm afraid novice plasterers panick too much and worry the gear will go off before they get it smooth. They keep messing with it just dragging it around the wall trying to get it smooth.

keep practicing. you will get it in the end then you will have a big grin on your face.
Hey all,

Sorry to bump this item back up the forum list but I really wanted to say thanks again for all the advice and give some feedback for anyone else in the future who might be experiencing the same issues I was.

I slowed my mixer right down, and gave my coats a little bit more thickness which seemed to work.

Also I let the plaster go off a bit more than I ever had before and just worked is smooth with a bit of extra elbow grease!!

Finally, and most significantly I think, I really restrained myself with the amount of water I was putting on the walls during the wet trowels and that's made a world of difference. I only brushed the trowel with water rather than flicking/painting any water onto the plaster.

I've attached some picture below of my latest attempt (after 24hours drying). I know I'll never be a pro, but not a bad afternoon's work for an accountant!! :LOL:


I think that once you have laid it on you are going back on it to quick.. As for the mixing it, we mix our skim bag at a time in large gorrilla buckets and we only keep it on slow speed till there's enough stuff in the bucket to stop it splashin' every where, then quick as you like. We leave it after it's mixed for a minute or two before using it.Try not to put any water on the wall ,only on your trowel ;)

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