Rendering a fireplace

[MadAlicesDad]

I'm going to fit a woodburner and have had an installer round to quote for the job. The fireplace is opened up, and is stripped back to the bare, but sooty, stone and bricks. The installer reckons the best finish on them would be render. I have done rendering in the past but always outside in garden/garage walls etc, never in this situation. I thus have several questions....

What sort of preparation should be done on sooty bricks and stone before rendering?

The back of the fireplace is very uneven where it has been hacked away to fit other fires over the years - for example there is a large indent that is 2 inches below the original level - I guess I should fill this first and then render over it?

What technique should I use? - several thin coats or one thick one?

What strength mix should I use?

Answers to any or all of these questions will be greatly appreciated. Alternatively, is there something other than render I could use, e.g. a heat resistant plasterboard?
Mike

 

[MadAlicesDad]

Would something like this
http://www.british-gypsum.com/products/plasterboard___accessories/gyproc_fire/gyproc_fireline.aspx
or this
http://www.british-gypsum.com/products/glasroc_specialist_boards/glasroc_multiboard.aspx
be suitable instead of render in a fireplace that will have a woodburner in it?
Mike

 

[agbagb]

Glad to see the jobs coming along Mike.
I rendered mine initially. If I remember right filling the deep bits first so it was roughly flat, then a skim about 1/2 inch, 5:1 ratio with plastersizer. BUT! I did get some cracks in it, I think due to the brick work drying and salts building up, some areas sounded a little hollow.

I prepped the brick by giving it a good dry scrub, very messy, followed by a coat of watered down PVA.

A few years latter before I fitted the fire I've tiled it with slate. It looks good and is hard wearing. Piling logs up against it doesn't show any marks.

Sorry, I don't have any experiance of the boards.

Andy

 

[stuart45]

Paraffin is good for disolving soot. Dub out any low spots first. Use a couple of thinner coats, rather than one thick one. Lime mortar is better than cement mortar where heat is involved. You could use a 1/2/9 cement/ hydrated lime/ sand mix. You could also use 1/3 feebly hydraulic lime/sand mix. Its better if the top coat is weaker than the base.
The boards are designed as resistance in a fire time wise to things like steel columns. I don't know how they would react to being continually heated.They might be OK.
I am not a plasterer by trade, so if you get different advice from one of the spreads take theirs.

 

[SUPERSLASHER]

I did one a few years ago and it is as good as the day it ws done. First you will have to wire brush off the soot and any old brick and pointing. Then wash it down with a stiff brush and water. Put a coat of watered down pva on. Dub out all indents to the same level as the brickwork. Then apply 2 scratch coats of render i.e.4 sand 1 lime i cement and Opti-rend render additive. Leave for 24 hours and then apply the finish coat of render i.e.6 sand 1 lime and 1 cement with additive added. Rub up with a wooden float to a fine finish. Use only washed plastering sand not building sand. As for the board i have used it only on steel as a fireproofer.

 

[MadAlicesDad]

First of all, Happy New Year to everyone, I hope you had a good break. Anyway Christmas is over so it's back to the jobs.
I did the first coat this morning. Got rid of the soot and as much of the old render as possible (it is difficult to tell where render stops and the old mortar begins!). I screwed stainless mesh across to give something to bond to, especially as there was a big hollow in the middle (approx 3 inches below the nominal stone level).
I used a 9/2/1 mix as suggested by stuart45 and it went on like butter! just about the easiest render I've ever applied, even with the uneven surface and crumbly mortar.
It will need another scratch coat before then final coat as there's still a 1.5 inch depression to fill in one place, but I'm a bit more confident about it now - thanks for suggestions and advice so far.

PS it's a bit late for me now, but what is the benefit of washed plastering sand rather than (washed) builders sand, and what does Opti-rend do?
Cheers,
Mike

 

[SUPERSLASHER]

Washed plastering sand has more grit in it so it stronger that building sand .Opti-rend render additive it is a plasticises waterproofers ******* and stops salts.