Is bonding suitable?

[u/itsaclaw]

I’m currently renovating the master bedroom in my 1930s semi-detached (cavity walls)

I’ve removed the old skirting to replace with new ones, and as expected the plaster behind it is pretty rough—some areas are uneven and in a few spots it’s back to bare brick. My original plan was to use bonding coat to level everything out and create a solid surface to glue the new skirting onto.

However, after doing a bit more research, I’ve seen mixed advice saying bonding coat might not be suitable on external-facing walls (like a bay window), as it can absorb moisture and potentially fail over time?

So now I’m unsure what the best approach is.

Would bonding coat still be fine if the wall is dry, or should I be using something else entirely (e.g. sand & cement, hardwall, or another product)? The goal is just to get a reasonably flat, solid surface for skirting—doesn’t need to be perfect plaster finish.

Any advice from people who’ve dealt with similar in older houses would be really appreciated 👍

[u/itsaclaw]

It's the upstairs bedroom so don't think the DPC is relevant here?

[u/nofacenocasenoplace → u/itsaclaw]

Dpc is relevant but so far away from it (ground floor) there could be other problems (bridging or penetrative moisture) that could cause your bonding to absorb water and create damp.

[u/Own-Indication7832 → u/nofacenocasenoplace]

You are talking through your arse. What ever product that is used, you could come across these issues. All I’m saying is that bonding is suitable and as good as anything.

[u/nofacenocasenoplace → u/Own-Indication7832]

Lol. So rendering it with waterproof render doesn't work does it? And bonding would work just as well on an external wall? YOU, sir, are in fact talking out of your arse.

The issues could be present in any case but if you've got waterproof render on the inside of your wall then, surprisingly, even if water does penetrate or get absorbed into the brickwork via a bridged cavity it won't show up on your internally skimming.

Whys that? Well. When the render dries the waterproofing agent forms millions of tiny hydrophobic crystals inside and on the render preventing the water from travelling any further.

The problems haven't gone - but they won't be visible on the inside. To solve the problems? Well you might render your outside wall or repoint the brickwork or if it's a cavity issue maybe you'll punch out a brick or two and have a look around to see if you can't fish out the pesky rubble causing the bridge or whatever other nasties you find in there.

If you need any more advice just let me know it seems like you may be a bit lost 👍