Using Bonding Coat to flatten ceiling?

by Pas
(London)

Hi,

My Starter Home was built in the late 80s, all the internal walls and (artex) ceilings are plasterboard, including the bathroom.

The bathroom ceiling is in two sections; one flat, the other part of the roof - sloping.
This sloping section was made out of two plaster boards, with the second. filling the 30cm or so gap (to the wall).

The original artex hid a slight step between the boards, now it's scraped off I've noticed the imperfection and some very shallow high and low points as if there is a slight bow.
I want to level the bottom section beneath the step.

I have replaced the jointing tape between the boards and added the joint compound ( also to all wall and ceiling joins).

My plan was to level it all out with the backing coat and floating method but am confused as to what is the best material to use?

I've brought a bag of ready mix sand and cement, but not sure if this will stick?
I thought about bonding coat but because it's the bathroom, I didn't think you could use it (damp room)?

Please can you advise.
Thank you and regards,

Pas.

P.s - great site that I've learnt a great deal from!


Answer

Hi Pas,

Sand/cement wont bond well enough - unless well prepared with PVA slurry (but best to forget about that)

The best plaster to use will be Bonding Coat as you mentioned. It is absolutely fine to use in bathrooms - but you are right - not good in damp situations.
to clarify this further - damp in this instance would mean it is - continuously damp. (For example if the wall was continually damp due to moisture penetrating then Bonding Coat would not perform well.)
But a bathroom ceiling that is just subjected to a bit of steam is no problem. To put it into context - standard plasterboard is the same - this will also not withstand continued dampness but is fine on a bathroom ceiling.

One coat of bonding coat over your existing plasterboards (sealed with PVA if in doubt) can then be ruled flat with a straight edge.
The Bonding Coat will set within 2 hours so ensure before it does that there are no lumps or ridges that stick out more than 2-3 mm otherwise your skim coat will have to go on thicker than normal in order to cover them. Any such highs in the Bonding Coat should be flattened when it has firmed up enough so that you can knock back any highs but it will not be that soft that it gets pushed out of shape. Flatness is always the first priority. It's all about timings - catching it at just the right time!

Regards
Paul