plastering: first attempts

[Fred]

If I had done half of the wall, it may have looked a bit funny ;)

Now that the plaster is dry I have been able to have a closer look at it.

I had skimmed over a brand new plasterboard wall and a wall which had been skimmed, painted, and papered previously. I had removed the wallpaper and in places the paint had come off with the paper, so it was a mixture of painted plaster and bare plaster; I PVA'ed all of it.

The plasterboard wall seems to be better. I had heard that PB was easier to do. I wonder why? Perhaps it's because it is perfectly flat to begin with? OTOH I don't know how flat the other wall was because I never thought to look beforehand.

I hadn't mixed plaster before so I had one batch that was too runny: I got more on the floor than the wall! I overcompensated on the next batch and it was too thick. Consequently I think it set quicker and was harder to work with. I think this stiff mix is responsible for some of the undulations. It's a shame I have only noticed them now, rather than at the time.

One web site I read talked of the plaster not falling off the trowel but I now think that is too stiff. I think it is right when it slides slowly. Do you agree?

Are professionally done walls completely flat? If you look at my wall, you cannot see any obvious bumps; it's not like parts stick out inches, so that's something but if you run your hand over it, you can feel slight rises and falls in places. Hopefully no-one will go hugging the walls to find out.

There are one or two rough patches which I am more concerned about. What's the best way to smooth them out? I did try light sanding with a

120 grit paper. I worried that a coarser grit might damage the plaster but the problem is that the paper clogs almost straight away. Should I use a different grit or is there something more appropriate than sandpaper?

Thanks again.

PS I forgot to say that I had a patch where the skim coat of old plaster had come off the wall. I did wonder whether I should fill this before skimming but decided to carry on regardless. Even though I PVA'ed the patch, it still caused the plaster to dry and crack. Should I have used more PVA or should I have polyfilla'ed the hole first?

[Fred]

Thanks. I hadn't realised Makro were still about. I thought they had been closing branches?

That's the thing that puzzles me. Once the (finish coat) of plaster is on the wall, how do you know you have a uniform thickness and not 8mm on one end and 2mm on the other?

My walls seem to be horrible sandy browning with skim on top. It seems that people just keep skimming on top of the previous layer. I know we are only talking a couple of mm each time but does there ever come a point where you should strip off all the skim back to the browning? After all you don't paper over paper or tile over tiles.

What if you chiseled off the whole lot? How easy is it to plaster a bonding coat to masonry? Not that I'm about to try it! Are there different types of bonding coat? I won't be using browning that's for sure!

Thanks.

[Fred]

[snip]

Although they are geared and slow spinning, I don't think they have a clutch do they? It's a shame because wouldn't a slow spinner with a clutch have served a dual purpose as a core drill?

The Wickes one was reduced to clear whenI saw it but that was a year ago, at which time I had no thought of ever trying to plaster.

I think it was a less colourful (grey) version of this:

For DIY levels of plaster mixing, the smaller paddle with a couple of

Toolstation sell a number with sizes from 60mm across to 120mm across:

wonder whether 60mm is too small? I tried the 120mm one but when I mixed too stiff, the resistance of the mix pulled it out of the chuck! The hex looks a bit rounded, whether it arrived like that or whether falling out caused that I don't know. Perhaps I will get a smaller one?

[John Rumm]

Well to an extent you don't. However uniform thickness is not the goal, a flat top surface is.

Well you can tile over tiles, and occasionally papering over linig paper works as well! ;-)

Normally when the skim is "blown" and falls off as you strip the paper, its a good indication it needs re-doing.

If you set guide edges, or form screeds carefully, then backing coat is not particularly hard to do. Browning is for use on higher suction backgrounds IIUC, and so would probably be better suited to traditional brick walls. Bonding would be better on thermal blocks and over expanded metal lath etc.

[John Rumm]

>>> For DIY levels of plaster mixing, the smaller paddle with a couple of

I think mine is 3 or 4" diameter...

[geraldthehamster]

Use a coarse grade glasspaper - 60 grit should be fine. Use it in a hand sander, so you can sand flat. Wear a mask ;-)

Cheers Richard

[stuart noble]

Screwfix decorator rolls are good for plaster

[Fred]

But couldn't you have a flat surface that was triangular in cross section if you put too much on one side? But perhaps I am being too hypothetical. I know my attempt wasn't perfect but it's not that bad ;)

But isn't that just a sign that the skim is bad? Why should the bonding coat need redoing, or is it just that you can't remove the skim without knocking off the bonding? I had a small patch of blown plaster that I didn't bother to fill before skimming. Surely for an odd patch of blown plaster, that doesn't require the whole wall to be redone? Is it only when lots of plaster becomes blown?

I did put PVA in the blown hole but even so the skim cracked. What did I do wrong? Should I have sued more PVA?Should I have filled the hole with polyfilla first?

Thanks.

[John Rumm]

Well you could, but would you care? Its the finish that counts generally.

In reality since you are only towelling on a few mm or plaster, its going to be hard to be that far out at one end!

Quite often the bonding coat is fine, and the skim just comes away from it.

Normally the base coat will stay put unless the wall moves or gets particularly damp.

Indeed - a blown patch of base coat can be hacked off and redone without touching the rest. (tapping it all over will normally highlight any blown areas - they sound hollow)

This was a hole in the skim? I find you need to give it a good wetting down, and a coat of PVA to stop the old plaster wicking all the moisture out of the new skim too fast.