Render the underside of an overhanging sandstone step

[Outer Bongolia]

I have a sandstone staircase from my 1st floor flat down to the garden, the top landing is about 90cm x 150cm and is positioned over the door of the downstairs flat. It's probably 100 years old and bits of the sandstone have been falling off so it's a bit of a safety hazard for the lower neighbour. I've removed all unstable pieces and steel brushed it back but it looks pretty unsightly and I'd like to render it back to a finished look. I'm not sure if a straight cement/concrete render will hold for long and wondering if a resin based compound might be more stable?

 

[RandomGrinch]

Could you bond/fix a bit of cement board to the underside instead?
It may be slightly more straightforward?