PVA and plastering.

[harrisonline]

Hi.

I am scraping some wallpaper off back to the ols plaster. I am going to skim with a fresh coat of plaster.

I want to paint the wall with PVA.

What I would like advice on is this.

Is it best to buy Unibond or are the shops own makes OK?

With what part of water should I dilute the PVA?

Can I do it the day before or should it be done just before the plastering?

Thanks.

 

[Richard C]

What type of paint is on the wall underneath the existing paper?

 

[harrisonline]

I think most/all of it is bare plaster. I haven't removed much of the paper yet but there may be emulsion in places!

 

[Richard C]

Make sure you remove all traces of paper or the skim will blister & wash all the old wall paper paste off the wall as thoroughly as you can. Depending on what’s on the plaster (if anything) will make a difference to how you need to prep the wall; why do you think it needs re-skimming?

It sounds like you haven’t done this before & plastering is not something that is particularly DIY friendly. It’s not just a case of buying a trowel, hawk, flexi bucket & some plaster from B&Q expecting to get good results the first time around; or even several attempts after that. It’s a real skill, some are born naturals but others will never be any good no matter how many years they try & believe me it can take years!

I would suggest you have a read back through the archive posts to try & understand what your letting yourself in for, buy a couple of boards, fix one to a timber frame & have a practice before you let rip on the wall itself. You may then decide to call in a plasterer but if you decide to have a go at least it won’t come as a complete shock the first time out!

 

[roy c]

If you can get the "real" unibond get it, thin it down at 3or4-1 get yourself a roller and tray and roll it on your walls the day before you are skimming. The next day give it another rollering before you mix your skim and then just skim on to it as normal....Sometimes depending on the back-ground you can add bonding (like roughcaster does) to the unibond or sand.But what you say I think you will be ok with "neat" unibond.....Good Luck...

 

[harrisonline]

Thanks.

I'm not doing it myself.

I usually have the same plasterer to do my skimming for me and he takes care of the lot. I just let him get on with it.

But I have been pressured into letting a young family member do it for me this time to help him out. He has done a great job for other family members and is a lot cheaper than my usual plasterer. He just lacks the experience.

The problem is that I have to go out and get all the materials and he wants it Unibonded the day before. He told me to get a gallon (4.5 ltr) of unibond but the area is only about 10 - 12 square meters.

 

[Richard C]

Must admit I don't use the expensive branded stuff; I've always used B&Q red PVA & never had any problems. If the wall’s painted, a 3:1 mix the day before should take care of most eventualities as Roy says but if it’s bare plaster, you may need an initial coat of 5:1 mix. Vinyl or gloss paint really needs surface scoring or priming with a PVA & cement/sand mix or even one of the proprietary, low absorbency primers. Your man will need to give it a going over with 5:1 mix (& let go tacky) before he plasters the wall though; lets hope he's as good as your relatives say he is

 

[harrisonline]

Thanks.

I have now removed most of the paper and I have a good mix of bare plaster in some areas, cement in others and some sort of emulsion. Isn't there a magic ratio that will work on all of it?

 

[walltowall]

3 parts water 1 part pva mate

 

[trowelmonkey1]

thsi PVA debate can go on forever and probably will, my tuppence worth will only go to add to the confusion but here goes:

5:1 for both coats is what i was told and have always done up til now, first coat 24hours before, 2nd before skimming, was also told that if the surface was painted that that effectively did the same as the first coat of PVA and controlled the suction, i have since had a conversation about the first coat being for adhesion as well and not just suction control.

seems like a bit of a minefield.

 

[Richard C]

5:1 for both coats is what i was told and have always done up til now, first coat 24hours before, 2nd before skimming,

5:1 is fine if you just need to control suction, it’s basically water with a little PVA in it to give it a little extra bite. If just controlling suction, you don’t need to apply the 1st coat 24 hours before, it will achieve virtually nothing. Just apply 2 PVA coats in quick succession when you start; in all probability the 1st will get sucked in straight away so apply the second which should remain on the surface much longer & then start plastering as it goes tacky; if the second coat gets sucked in as well, you may need a 3rd coat!

was also told that if the surface was painted that that effectively did the same as the first coat of PVA and controlled the suction, i have since had a conversation about the first coat being for adhesion as well and not just suction control.

Matt emulsion is usually OK but if the original surface has been painted with vinyl emulsion or gloss paint, there will be no suction! Good news as the plaster will remain workable much longer but the bad news is it will have little key on the surface; a 5:1 PVA mix will just run down the wall & achieve nothing, hence the need for scoring the surface & applying a strong, bonding coat of PVA the day before; which must be allowed to dry out. PVA remains water soluble so, a second weaker PVA applied just before plastering the next day will soften up the surface so it goes tacky & give much better adhesion; another method is to put a handful of sand or cement into the bonding PVA coat.

seems like a bit of a minefield.

It’s not that critical, you just need to understanding what’s going on; the last thing you want is for the life to be sucked out of whatever plaster your attempting to get on there so it goes off in 20 minutes or to have, maybe, an hour to work on it only for it to fall off in lumps when you knock it due to lack of adhesion!

 

[trowelmonkey1]

great and comprehensive response Richard, THANKS! that all makes perfect sense to me now, of course 5:1 PVA would run down a glossy wall.

have you ever considered using Gyprocs products in place of PVA? they seem like great alternatives but VERY expensive.

 

[Richard C]

have you ever considered using Gyprocs products in place of PVA? they seem like great alternatives but VERY expensive.

Honestly, I've never tried them or felt the need to; being a suspicious git, I suspect they are really just an expensive way of selling you PVA. As I said previously, I don’t even use the branded stuff, just “B&Q Red” which I’ve never had any problems with.

 

[trowelmonkey1]

i suspect that it does have some good properties and may well be better than PVA...BUT i'm with you, i've been using Sika PVA from B&Q which has served me well and is MASSIVELY cheaper, so long as you use it correctly etc i see no reason to change really

 

[jabaspurs]

Hi, we use the the B&Q PVA with no problems and my boss tends to put neat PVA on the walls/ceilings the night before. I've also worked with one group of sub contractors who water it down.

Anyone else use it neat?