How to deep fill an area under a stair winder?

[redhexagonal]

So I have a stair winder which is plastered in a haphazard manner underneath, its quite wavey, and I’m trying to neaten it up by shaving bits off and building it up with easifill. There are a couple of deep dips in the corners that could do with being filled out to a depth of up to two inches. Using easifill that deep doesn’t seem a great idea, is there a way I could build the area up with something before applying plaster? I was thinking maybe screwing something up there or maybe fashioning a wedge of expanding foam? The area is a right angled triangle with short sides of 40cm . The area to be filled is roughly the bit in shadow in this photo

 

[redhexagonal]

could I just key the area and fill it a couple of inches deep with bonding and then a thinner layer of easifill to get it smooth? or would that be too heavy

 

[Deleted member 292770]

scrape the paint back and/or scratch it all with a stanley blade first. then pva before bonding coat

 

[redhexagonal]

scrape the paint back and/or scratch it all with a stanley blade first. then pva before bonding coat

Thanks, will bonding be ok a couple of inches deep in one fill? And then easifill to smooth it off?

 

[Deleted member 292770]

yes, build it up. do say an inch and let it go off. goes off pretty quick . then do another coat.

 

[redhexagonal]

yes, build it up. do say an inch and let it go off. goes off pretty quick . then do another coat.

Do i need to put metal lath in it or anything to strengthen it?

 

[Deleted member 292770]

Do i need to put metal lath in it or anything to strengthen it?

no. getting a good key is the important thing.