Applying plaster for smooth finish to a bricked & vented in fireplace.

What plaster thickness are you talking about?

If you are new to this, and this is pretty much a one-of job, then you might be better off using one of these 'one-coat' plasters which can be applied pretty thickly, up to a good 10mm (although the thinner it's applied, the easier it will be to handle.)

Is this an internal area or external? If the latter, consider fitting plastic edging beads down the external corners to guide you.

PVA probably not needed - just make sure that the surface is not excessively absorbent; try a light water spray and see if the bricks, for example, immediately soak up the water whilst the mortar lines remain shiny, or vice versa. It can be a pain if one part really soaks up water when other bits don't - you'll find the plaster will dry in matching blocks...

If the bricks are just a wee bit absorbent, then simply lightly dampen the whole surface with your trigger spray before applying the skim.

If you are a complete newbie, then get hold of a length of timber batten with a crisp edge to use as a guide for levelling the skim after applying; shove the stuff on, get it reasonably level using your trowel, and then use the batten to confirm. Hold it horizontally, at right angles to the surface, and either drag it gently upwards repeatedly to 'trim' the high spots/fill the hollows, or else slide it right and left - gently - as you drag it upwards.

Once roughly level, use the trowel to get your final finish. Don't try and get it good too early - the skim needs to be beginning to set (go like thick plasticine) before you can apply enough pressure with your trowel to get it fully flat. Use your trigger spray to lubricate the surface as you do this.

 

I wouldn't try smoothing/finishing any plaster that's more than 5mm thick!

Mr. HandyAndy - Really

 

I think bonding coat is ok up to 10mm

 

I would look at using Thistle Hardwall undercoat plaster. Apply by trowel and use a batten as a guide and level off the plaster using the existing plaster as a gauge. Once level allow 1/2 hour to go off. Then use the trowel to scrape back by 2-3mm of plaster - once this is done 'key' the surface using the corner of a trowel to provide a scratch coat. Allow plaster to harden.

For finishing coat use Thistle Multi-finish.

 

There are products - I think called 'patching plaster'? - which can be applied over 10mm thick and will provide a final finish.

Not saying it's good stuff 'cos I chust don't know, but it sounds as tho' it could be chust what my fellow MacScot is after.

And it only needs to be applied to the thickness of the highest point on his wall, which may only be 6-odd mm?

Anyways...

 

Like this stuff: http://www.wickes.co.uk/Wickes-One-Coat-Plaster-25kg/p/221806

Do our hosts make an equivalent?

Gulp - can be applied up to 20mm thick in one application? Only if you're brave...

 

I'm no expert, but I think if you let the plaster dry overnight, you should PVA it before applying the top coat.

Mr. HandyAndy - Really

 

I think I found some of that stuff under the wallpaper on our walls when we moved here............... it was still soft! The wallpaper was not new.

Mr. HandyAndy - Really

 

The undercoat should be 'set' but not dry. If allowed to dry out completely it will create too much suction, and when applying the finishing coat will draw out the moisture causing cracking to occur. The area can be dampened down prior if there is a risk that this could happen.

The last time i did plastering, the undercoat was allowed to set for about 4 hours. It should be reasonably flat and keyed to allow a 2mm finishing coat to go on top - this is applied in 2 layers. The idea is to get the first coat on roughly level - don't waste time trying to level it out too much, that's for the 2nd coat. Also if the plastered area is going to be tiled don't' polish the surface, a matt finish is all that's required.

 

Note setting time:

British Gypsum Thistle Hardwall Plaster 25kg

• Setting Time: 1.5 - 2 h
• Life-span: 4 month
• Coverage: 2.25m² at 13mm thickness
• Usage: Suitable for all but smooth low suction backgrounds

 

The only time i use one-coat plaster is when I have cut channels for electrical cabling or I have deep holes to fill - though it is best to apply in layers rather than one, goes on a very easy and is much more denser than standard undercoat. When this has dried I apply pollyfilla to fill in any blemishes and then sand when everything is dry.

 

That sounds like a plan, JG - I'm assuming the OP just wants a simple, quick solution, and this what you suggest should provide it without having to buy 'bonding' and 'finish' and all that jazz.

Bottom line, even if it goes pear-shaped, the OP will still get it 'perfect' by using a guide to check the finish, sanding down any high spots and filling any low. A bit of labour .