Bonding agent for re-skim on old bonding plaster

[SteUK]

Hi all,

Wondered if I might get some advice on preparing an old bonding(?) coat for a re-skim of finish plaster.

Would SBR be suitable as a bonding agent?

Here is a pic of the bonding coat I will be going over.



Most of the current finish coat has blown and will be coming off prior to re-skim.

Thanks!

 

[roy c]

Yes SBR will be fine, what size area is it and are you going to tile on it again? If not you can sbr it let it dry then put another coat on it and let this coat go tacky before you skim it or bond it out,,,

 

[SteUK]

Awsome! Thanks Roy!

It's the long side of a bath wall. Will be cement boarding, ranking and tiling. Tiles are up at the 20kg/sqm mark with adhesive and grout added, hence the cement boarding. Will be fixing the boards with Mapei keraflex maxi and mechanical fixing (Fischer ux x 40mm plugs and 50mm stainless screws - would have used deeper but the wall is an internal 50mm deep only cinder block wall!).

 

[roy c]

If you are going to hide that wall I would just spread it with you tile adhesive, just go straight on to it and just get it as flat as poss, Don't bother with the sbr doing it like this,..

 

[SteUK]

Don't bother with the skim? Just attach cement boards straight to the bonding?

That was my original plan, so I contacted Mapei to ask if any of their adhesives bonded to bonding plaster. They said absolutely not...and I would have to finish coat the wall.

 

[roy c]

I didn't say apply straight to the bonding I said if you are going to cover the wall then just level it up with tile adhesive straight on to the wall.. I don't think that the plaster on your wall looks like bonding in the photo it looks like sand and cement and skim. Can you tell for certain that it is bonding? It has obviously been tiled on before and it is not good practice to tile onto bonding,,

 

[SteUK]

Interesting! Never thought it was anything but some old form of bonding under the skim. It's the same the whole house over (60's ex council house). Is there a lay mans way of telling what it is for sure? Or is it just an experience thing.

 

[roy c]

Get a screw driver and see how far you can push it into the plaster in different parts of the wall. If it is bonding or browning (both light weight base plasters) then the screw driver will go in easy, if its not any of those two it will be hard to push it in the wall..

 

[SteUK]

Starting to think you might be right Roy! Here are a few closer up pictures of a chunk of it...





If it is, I can just prime and tile-adhesive the cement boards directly onto it, right?

 

[SteUK]

Screwdriver makes a mark on the surface, and a little crackle noise under pressure, but doesn't even push in a mm.

 

[roy c]

You don't even need to prime the surface, you wouldn't prime it if you were tiling on it eh? So just level it out with your tile adhesive and when you put the boards on drill thru them and put your fixings and washers on . Good luck,,

 

[SteUK]

Thanks for your wise words Roy!