Skim coat cracks when drying out

[chrisjc]

OK, in view of Rich C's comments about using VITCAS near fires I will probably have to get a professional in. But for such a small area, which will be papered anyway, to either side of the wooden surround for a new cast iron Victorian fire insert (not a stove) I thought I would have a go with materials that were to hand.

So on Saturday I damped down the brickwork and put on a sand/cement render coat incorporating Febmix (over galvanised Expamet lath as the bricks were very sooty) with a diamond scratch pattern, and on Sunday a coat of ordinary gypsum plaster while the render was still damp, and achieved what I though was a satisfactory finish. This morning (Monday) it has fine cracks in it and is coming off the render in sheets.

I used the end of a bag of Multifinish and then opened a new small bag of thistle finishing plaster both stored in a dry cupboard, now I see the use-by dates were April 10 and November 09 respectively, is that the cause of the problem (the ordinary finishing plaster took for ever to go off, though the cracking is worse in the Multifinish)? Or lack of bonding agent or a bonding coat on top of the render? Or just poor technique?

TIA for any helpful comments

ChrisC

 

[Micilin]

I'll start the ball rolling.

You may have more than one cause here

- first off the plaster was too old

-when you put the plaster on did it 'suck' in very quickly? I'm assuming not as you said it took a while to go off and finish was okay

- did you trowel it more than 4 times?

-was 'forever' much more than an hour /hour and half?

It sounds to me like old plaster, just no goodness left in it. Possibly over- trowelled and 'killed' because it stayed soft for a long time.

You should not need any bonding agent onto keyed render, just got to be damp enough not to suck all the moisture out of the finish.

 

[Richard C]

The plaster has not bonded. You should never use out dated plaster or even plaster that’s in date once the bag has been opened for more than a few days; the stuff you’ve used could be anything up to 2 ½ years old. Even though the render is still damp, you still need to kill the suction & just water is best if the render hasn’t dried out. Multi that old is likely to go off in the bucket as soon as you start mixing it (or perhaps it won't go off at all, I've never tried it that old ) it will aslo craze like the devil; what make is the “finishing plaster” you’ve used, is it one coat? Did you put any lime in the render; you’ll need it to cope with the heat? You should also have used 2 render coats, the second slightly weaker than the first. I wouldn’t have much confidence on it lasting too long when exposed to heat & the Gypsum will fall off (if you can get it to stick in the first place) after a few weeks if the temperature in the wall exceeds 50 degrees on t a regular basis.

I checked & do not appear to have posted any of your threads personally but if you’ve read the advice/warnings in the links I regularly post about fires/stoves, rather strange that you should then risk it by ignoring them; as already posted, it sounds like you’ve made several mistakes. I assume you’re also aware of the B Regs regarding new & re-commissioned fires/stoves & the requirements for using non-combustible materials, ventilation, flues, notice plastes etc!

 

[chrisjc]

Looks like the age of the raw material was the worst problem. Behaviour of the two bags was different in detail, the Multi sucked quite badly and went off quickly but the finishing plaster (British Gypsum Thistle, not one-coat, see OP) took best part of 3 hrs to set.

Based on a very similar installation in our previous house I am not expecting temperatures anywhere near 50C outside the wooden surround. We will have to leave the job to dry out thoroughly and start with very small fires to temper the cast iron anyway, and only for show thereafter, we have just had all the central heating replaced.

What is the reason for applying the render in two coats? this is not how our damp-proofing contractors did it! If my wife wants to have another go over the existing render (she is better at the wet trades) do you still suggest just damping it or will it need a bonding agent now and if so what (e.g. we have some Unibond PVA, it suggests 5:1 dilution on the tin).

The proprietary CI insert and fireback will be installed with a metal "gather" in the builder's opening and a slate hearth slab in 4 pieces bedded with weak mortar on the existing constructional hearth with a metal fender round it. Maybe you have some comment on whether to seal the CI insert to the wall with fire rope or plaster, the suppliers suggest either? Am in general well conversant with the BRs and have done entire house rewiring self-certified to the surprise but complete satisfaction of BI. Good job electrical materials don't deteriorate like plaster in storage, in fact 30 year old cable is more flexible and easier to draw through holes than the stuff you can get now!

Many thanks

 

[Richard C]

but the finishing plaster (British Gypsum Thistle, not one-coat, see OP) took best part of 3 hrs to set.

But British Gypsum Thistle what?
Thistle One Coat Plasters
• Thistle Universal One Coat
• Thistle Projection
Thistle Undercoat Plasters
• Thistle Bonding Coat
• Thistle Hardwall
• Thistle Tough Coat
• Thistle Browning
• Thistle Dri-Coat
• Thistle X-Ray
Thistle Finish Coat Plasters
• Thistle Multi-Finish
• Thistle Board Finish
• Thistle Durafinish

Based on a very similar installation in our previous house I am not expecting temperatures anywhere near 50C outside the wooden surround. We will have to leave the job to dry out thoroughly and start with very small fires to temper the cast iron anyway, and only for show thereafter, we have just had all the central heating replaced.

If that’s the case, you’re probably OK & I suspect there is some leeway in that 49 degrees maximum but I don’t know for sure.

What is the reason for applying the render in two coats? this is not how our damp-proofing contractors did it!

It will reduces the chance of cracking, especially important where high heat is involved; but if you don’t think your in that category, hardly worth doing it now. You should always add lime where heat is concerned but it’s a good idea in a render base coat anyway.

If my wife wants to have another go over the existing render (she is better at the wet trades)

Blimey she sounds interesting, wish I had one of those

Am in general well conversant with the BR and have done entire house rewiring self-certified to the surprise but complete satisfaction of BI.

Many are PITA but BR’s should always be adhered to if you want to avoid problems in the future; more important now than ever.

Good job electrical materials don't deteriorate like plaster in storage, in fact 30 year old cable is more flexible and easier to draw through holes than the stuff you can get now!

Trouble is it’s the wrong colour unless you’re trying to Pull a fast one

 

[1john]

You say that you scratched the render with a diamond pattern, this seems as if that practice would normally be quite a deep key which would take a second coat of render which would only be scratched with a devil float prior to skimming, how thick did you apply the plaster and how thick was the mix?

 

[chrisjc]

But British Gypsum Thistle what?
Thistle One Coat Plasters
• Thistle Universal One Coat
• Thistle Projection
Thistle Undercoat Plasters
• Thistle Bonding Coat
• Thistle Hardwall
• Thistle Tough Coat
• Thistle Browning
• Thistle Dri-Coat
• Thistle X-Ray
Thistle Finish Coat Plasters
• Thistle Multi-Finish
• Thistle Board Finish
• Thistle Durafinish

None of the above. The bag just says Thistle Finishing Plaster - Finish Coat for Undercoat Plaster. It is a 2.5kg retail pack AFAIR bought at B&Q for a previous patching job but never used. The detailed instructions on the bag say it is suitable for both boards and undercoats so it might be Multifinish inside but it sure behaved differently from the other bag.

You say that you scratched the render with a diamond pattern, this seems as if that practice would normally be quite a deep key which would take a second coat of render which would only be scratched with a devil float prior to skimming, how thick did you apply the plaster and how thick was the mix?

The mix was creamy like it says on the tin, it went on fine, thickness a bit variable say 2 - 4mm, we only are talking about strips 6 ins wide so I was doing the same as I have used to fill many conduit chases in the past with satisfactory results (see electrical bit below).

If my wife wants to have another go over the existing render (she is better at the wet trades)

Blimey she sounds interesting, wish I had one of those

Wife says she will now do it. We only need a finish suitable for papering so I wondered about using One Coat especially if it can be applied in a greater thickness, do you have any comment on this and the seal to the fire insert, and should she put any bonding agent on the render (see previous post).

Good job electrical materials don't deteriorate like plaster in storage, in fact 30 year old cable is more flexible and easier to draw through holes than the stuff you can get now!

Trouble is it’s the wrong colour unless you’re trying to Pull a fast one [/quote]

Well actually if the colours are mixed because there is both old and new cable then under Part F you have to put a warning notice on the consumer unit. Once you have done that who is to say which wiring is "new"?? Also if you are doing a quick surface job (yes, they're still allowed) say an extra socket in the lean-to for the freezer then it's far better looking in white cable (and cleats) but it's almost impossible to obtain now so I keep reel ends for that kind of thing, and old MK and Crabtree fittings too so I can fit them to match existing. BTW have just bought a new Bosch freezer and they say they are not warranted to work in an unheated room below 10C, how daft is that?

Thanks again

 

[Richard C]

The bag just says Thistle Finishing Plaster - Finish Coat for Undercoat Plaster. It is a 2.5kg retail pack AFAIR bought at B&Q for a previous patching job but never used. The detailed instructions on the bag say it is suitable for both boards and undercoats so it might be Multifinish inside but it sure behaved differently from the other bag.

Didn’t know they sold it like that but I only ever buy 20kg bags; lots of em! From what you say & the fact it took so long to go off, my guess would be its one coat but it’s age could be a big factor.

Wife says she will now do it. We only need a finish suitable for papering so I wondered about using One Coat especially if it can be applied in a greater thickness, do you have any comment on this and the seal to the fire insert, and should she put any bonding agent on the render (see previous post).

Personally I hate one coat but it has its uses I suppose. Use 1:4 PVA/water mix on the render, let the first dry, then plaster as soon as the second has gone tacky. I’m still concerned it won’t stand the heat but I guess you’ll find out soon enough. Silicone is as good as any to seal but don’t get it all over the wall as not much will stick to it.

Well actually if the colours are mixed because there is both old and new cable then under Part F you have to put a warning notice on the consumer unit. Once you have done that who is to say which wiring is "new"??

Also if you are doing a quick surface job (yes, they're still allowed) say an extra socket in the lean-to for the freezer then it's far better looking in white cable (and cleats) but it's almost impossible to obtain now so I keep reel ends for that kind of thing, and old MK and Crabtree fittings too so I can fit them to match existing.

As I said, your pulling a fast one; mixed colours are allowed with a warning notice as you say but that only applies to the original wiring, not new circuits or spurs. What does Part F (Ventilation) have to do with it , its Part P your concerned with & this states;

“The new (harmonised) colour cables may be used on site from 31 March 2004. New installations or alterations to existing installations may use either new or old colours, but not both, from 31 March 2004 until 31 March 2006. Only the new colours may be used after 31 March 2006.”
Using original coloured cable on new work is a really bad idea for DIY, it’s not in accordance with BR’s but if you’re doing paying electrical work with old cable colours your breaking the law I believe; & as for using old fittings

BTW have just bought a new Bosch freezer and they say they are not warranted to work in an unheated room below 10C, how daft is that?

That’s very common now, something to do with efficiency & external ambient temperature sensors I believe but not really sure.

 

[chrisjc]

Many thanks for yr help

Didn’t know they sold it like that but I only ever buy 20kg bags; lots of em!

And I thought the standard bag was a nominal 25kg.

What does Part F (Ventilation) have to do with it , its Part P your concerned with

Realised this as soon as I pressed "Submit"... Part F is on my mind because I am waiting for parts for a heat recovery system at the moment. Never really got used to the change from IEE Wiring Regs - nor for that matter their rebranding as IET for all other purposes.

 

[Richard C]

And I thought the standard bag was a nominal 25kg.

Your right, they are 25kg