Re-skimming – Does the whole wall need to be bonded/hardwall

[benjiman]

I had a plasterer come around to give a quote to re-skim two rooms. The existing skim on the walls in really flaky. Some of it just comes off with a scrapper.

I’ve taken all the loose bits off, the rest of the wall is now sound. No hollow sounds, or skim flaking off. Underneath the skim is a dryish yellowish undercoat. It is a bit powdery but its stuck solid to the walls.

The wall has some areas where there is skim on it as its stuck well and others where it has been taken off completely to the backing coat. I’d say its 80% taken off with the remainder is patchy skim.

The plasterer has said he needs to put a thin layer of bonding/hardwall over the whole wall. Then he will put the skim coat on top of this. He said if you put it direct onto the wall the skim coat will crack (I mentioned PVA to him, but he said that doesn’t work, eventually it will end up cracking due to heat etc) He said if you don’t do this then the next time you take the wallpapers off the skim will come away from the wall, just like it has done now.

He said most skimmers don’t do this as its too much effort but its the correct way to do it. He also said on other jobs that he does he puts the bonding on, then some kind of plastic mesh is pushed into the bonding. Then he skims on top of this. This way he claims the skim never cracks.

Is he correct with this advice?

 

[NickB_99]

Is he correct with this advice?

Only bits of it sound right to me.
If it's uneven, a tight coat of Bonding to level it up sounds sensible. However, this must have diluted PVA underneath or at least well wetted or it will just crack and come away as too much suction.
If done the same day, can probably skim over with no prep between the Bonding and multi-finish (2 thin coats).
I've never heard of the plastic mesh pushed in across it all. Local patches yes, where cracking has happened or with metal lath more usually than a plastic mesh. I would imagine it may help strengthen a bit, but would be more in terms of time & materials. May also reduce the bond between the Bonding and underneath as possibly less surface area contacted effectively. Not sure if the gain is worth it.
I don't think this will allow a guarantee it won't crack. If you get movement in bricks underneath, scrim/plastic mesh etc in the plaster will not have enough strength to be impervious to cracking.

Pros may be able to help you a bit more.

 

[WhatsHisFloat]

Tight coat of backing plaster to dub out to existing level and reskim is fine however may crack where new backing plaster meets existing...

so a tight coat over the whole lot is fair enough, its only to provide a level and a mechanical key. In which case, it may be just as easy to hack off the the remaining 20% that is left on the walls and re-float the whole wall.

Either way i wouldnt disagree with what he has said, although others might )

Good luck

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

 

[benjiman]

Tight coat of backing plaster to dub out to existing level and reskim is fine however may crack where new backing plaster meets existing...

so a tight coat over the whole lot is fair enough, its only to provide a level and a mechanical key. In which case, it may be just as easy to hack off the the remaining 20% that is left on the walls and re-float the whole wall.

Either way i wouldnt disagree with what he has said, although others might )

Good luck

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

Thanks WhatsHisFloat. When you say re-float the whole wall do you mean bonding on the whole wall and then skim? Or skim directly on top of the old backing coat?

 

[benjiman]

Is he correct with this advice?

Only bits of it sound right to me.
If it's uneven, a tight coat of Bonding to level it up sounds sensible. However, this must have diluted PVA underneath or at least well wetted or it will just crack and come away as too much suction.
If done the same day, can probably skim over with no prep between the Bonding and multi-finish (2 thin coats).
I've never heard of the plastic mesh pushed in across it all. Local patches yes, where cracking has happened or with metal lath more usually than a plastic mesh. I would imagine it may help strengthen a bit, but would be more in terms of time & materials. May also reduce the bond between the Bonding and underneath as possibly less surface area contacted effectively. Not sure if the gain is worth it.
I don't think this will allow a guarantee it won't crack. If you get movement in bricks underneath, scrim/plastic mesh etc in the plaster will not have enough strength to be impervious to cracking.

Pros may be able to help you a bit more.

Thanks DIYnewbee99, makes sense to me. The plastic mesh thing was the 1st I've heard too!

 

[NickB_99]

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

That's a really good summary. Fancy adding it to the Plastering Wiki?

 

[benjiman]

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

So is it advisable to use a bonding agent for a more reliable finish against cracking? I presume you still pva the wall and then put on the agent and then skim on top of that?

 

[peaps]

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

So is it advisable to use a bonding agent for a more reliable finish against cracking? I presume you still pva the wall and then put on the agent and then skim on top of that?

PVA is a bonding agent but it's water based and no good in certain situations. If you go with thistle bond-it you have no need to use PVA.

 

[benjiman]

peaps - just been reading about Wickes bonding agent...reading good things about it.

So basically its just brush on I believe. Question is can it be used on an old backing coat? The backing coat is solid, stuck well to the wall, but it is a bit powdery. Can it be used on this?

 

[WhatsHisFloat]

PS: Just a point to mention about PVA, it is used to control suction, to prevent the moisture in the newly applied plaster from being sucked into the background substate too quickly, if this happens, the plaster becomes unworkable very quickly, cures too quickly and cracking can occur.... PVA is NOT a bonding agent, something like Wickes WBA, Bluegrit, Bettakontact etc create a textured surface, not unlike very rough sand paper that provides a mechanical key for the skim coat to bond too... unlike PVA these Bonding Agents do nor re-emulisfy when wet

So is it advisable to use a bonding agent for a more reliable finish against cracking? I presume you still pva the wall and then put on the agent and then skim on top of that?

bonding agents provide a key that prevents the skim coat shelling off of the existing top coat - perfect example, you want to reskim a previously skimmed and painted wall.. skimming over the shiney painted surface, the skim is not going to bond to the painted wall... bonding agents effectively create a key for it to stick too.. you could of course scratch up the surface with a stanley knife but a paintable keying agent is less crude...

you do not need to use things like bonding agents on backing plasters, One of the ideas of backing plasters is to provide a keyed surface... if your existing background is very crumbly, id knock it off, if your fine with it, id dub out the whole wall to the same depth AND Then run a tight coat over the top so that the whole lot has a thin cover, skim directly onto that.

 

[peaps]

peaps - just been reading about Wickes bonding agent...reading good things about it.

So basically its just brush on I believe. Question is can it be used on an old backing coat? The backing coat is solid, stuck well to the wall, but it is a bit powdery. Can it be used on this?

Yes you can use it in this situation. I use a roller.

 

[benjiman]

Thanks for your help WhatsHisFloat and peaps. Much appreciated.

 

[WhatsHisFloat]

Let us know how you get on