Heat Resistant plaster advice please

[macstar]

Hi all

I need some advice for plastering around a hole in the wall fire. The plaster needs to have a heat resistance of 70 degrees so ordinary plaster won't do. I've been looking around this site and the web and have seen the Vitcas heat resistant screed and plaster. However this plaster can only be used around the fire and as the whole chimney breast would need skimming this won't do either as we want a nice smooth finish. I have read about using a 5:1:1 sand, cement, lime render then having a lime finish using lime putty mixed with silver sand at a ratio of 1 sand to 1.5 lime putty. Has anyone any experience of using this? and would it be heat resistant enough as I wouldn't want it cracking or falling off? Also can you use normal emulsion on lime plaster?

Thanks

 

[Richard C]

If you’ve a hole in the wall fire you may find the temperature can easily exceed 70 degrees C; I’ve measured temperatures of over 95 degrees C in the wall surrounding my own fire.

Rather than use Vitcas heat proof render, I’ve used 5:1:1 sand/cement/lime render which, properly applied, will withstand the heat. You can get a perfectly smooth finish with Vitcas plaster, it does behave slightly different to conventional finishing plaster & you must finish “off the trowel” as it dries very hard & can’t be sanded or blended in.

You don’t have to use Vitcas on the whole chimney breast unless the heat will exceed 50 degrees, just use it on the render immediately around the fire; 800mm above & 300mm either side of the opening is usually enough. Once you’ve rendered, apply the Vitcas plaster up to a stop bead, remove the beading when dry & then plaster up to this & blend in with gypsum finish plaster. Regards using lime putty, one of the other spreads with such experience may be able to advise as, personally, I’ve never used it but it’s a bit fussy & I can’t see any real advantage of using it over Vitcas plaster; it’ll probably be even more expensive as well.

Don’t know how many of the archive threads you’ve read but here’s a link to a recent thread which, in turn, has links to other threads;
//www.diynot.com/forums/viewtopic.php?t=249454&highlight=

 

[plstering]

http://www.british-gypsum.com/literature/fire_book.aspx

 

[macstar]

Once you’ve rendered, apply the Vitcas plaster up to a stop bead, remove the beading when dry & then plaster up to this & blend in with gypsum finish plaster.

Thanks richard. Will you not be able to see the 'join' between the two plasters or a difference in the surfaces?

 

[Richard C]

Will you not be able to see the 'join' between the two plasters or a difference in the surfaces?

Not if it’s properly done; for various reasons I’ve used removable stop beads to leave a plaster line which I’ve then finished up to from the other side sometimes long after the original finish has gone off. The Vitacs stuff behaves slightly differently to Multi finish, takes longer to go off & trowels up slightly differently but I find no texture difference between the two when it’s all finished. Multi is not that easy to blend in either for the inexperienced but any slight imperfections can be filled & sanded.
You might find this useful;
http://www.euroheat.co.uk/images/product_images/1710243172TB203 Heat proof plaster.pdf

 

[macstar]

many thanks richard for your help

 

[spc]

i hate to sound like a broken record but haired lime render whether it be putty of a feeble hydraulic (nhl 2or maybe 3.5) has excellent heat proof qualities, sponged up or if a finer finish is required a 50/50 kiln dried sand and lime putty finish. one it has carbonized its chemical composisition is reverted back to calcium carbonate, essentially lime stone which does the job. I know i sound like an old fart but why fight something that has worked for centuries

nb. a good way to work out the lime sand ratio is to measure a sample of aggrigate in a container, then add water till it comes to the level of the sand. the ammount of water it takes to fill the voids in the aggrigate is the ammount of lime you should add. then step up the quantities to suit the size of your project.

hope this helps.

 

[Richard C]

some technical stuff about lime putty

I’ve not heard any partially broken records extolling the virtues of haired lime render be it putty of a feeble hydraulic (nhl 2or maybe 3.5or sponged up 50/50 kiln dried sand and lime putty finish.

Good info for the lime putty experts (which I freely admit I’m not) & I can get the jest of it but, get real, this bloke just wants to plaster his fireplace & not have it fall off!

 

[spc]

it wont, lime flexes cementitious or gypsum doesnt..... simple.

Its not rocket science, just a different material. you dont need to be an expert, if you can plaster you can plaster.

And the reason i didnt want to sound like a broken record, read my posts, most suggest using lime

 

[Richard C]

it wont, lime flexes cementitious or gypsum doesnt..... simple.

I know that.

Its not rocket science, just a different material. you dont need to be an expert, if you can plaster you can plaster.

I know that as well.

And the reason i didnt want to sound like a broken record, read my posts, most suggest using lime

I hadn’t noticed that.