Plastering Fibre board?

[stevegreen]

Hi, can sand and cement scratch coat be succesfully applied to heat resistant cement fibre board, for behind a wood burning stove.

 

[Richard C]

I’ve not tried that one but what cement fibre board is it? When I first started finishing around stoves I had a catastrophic failure with Gypsum around my own so contacted Knauf about using Aquapanel around stoves. After consulting with their tech team they came back & said a definite no but they may have just been covering their asre. I use sand/cement/lime render as a base topped with heatproof plaster instead.

They also say that you can’t plaster Aquapanel except with their “special & very expensive” plaster but I have. I use it with edge beading for making bathroom access panels & skim the face with Multi after a good sealing dose of 1:1 PVA let dry for 24 hours; not had any fall off yet.

You can only but try I suppose but you may have problems getting it to stick if you kill the all suction with PVA.

 

[stevegreen]

Where can i get heatproof plaster from? Could a good plasterer get a reasonable finish with sand,cement and lime mix and not bother with heat proof plaster and would this withstand the heat by itself, or is the heat proof plaster required to protect the backing coat.
Thanks

 

[Richard C]

Render can be finished to look good but no matter how good the spread it's still going to look like render, very rustic; it will never polish up like finishing plaster. The sand/cement/lime render should be OK on it's own

Heat proof plaster;
http://shop.vitcas.com/vitcas-heat-resistant-plaster-16-p.asp
It's only available from the one source as far as I know, at least I've not managed to get it anywhere else.

It goes on & polishes up like Multi but takes far longer to go off; probably at least twice as long & you have to get it right "off the trowel" because it sets rock hard & can't be sanded at all.

 

[stevegreen]

I've had a look at the fire resistant plaster on vitcas's site and they say to apply about 6mm thick directly to blockwork - but if i do this at that thickness it could get expensive! would it be feasable to apply at half the thickness onto S/C/Lime scratch coat?
thanks

 

[Richard C]

For sure the bespoke stuff isn’t cheap but old fashioned sand/cement/lime mix stands up just as well as a base & I just put a single 2mm heatproof plaster skim over that; should be OK.

But get the "trowel" finish right cos you won't be able to put it right after.

 

[stevegreen]

Would you use standard ratios of sand/ cement/lime or use more lime?

 

[Richard C]

Have aread here;
//www.diynot.com/forums/viewtopic.php?p=1264123#1264123
//www.diynot.com/forums/viewtopic.php?t=211524
I used 5:1:1 - two coats, first one fairly wet but there are some other good pointers in there.