Can you vandex over blakjack?

[Dan1million]

Hi All, 

 

I'm starting my first semi-renovation project. it's a stone-built cottage on a hillside with the living room semi-subterranean - at it's highest point the road outside is about 1.2m above floor level.  I've removed the old decaying render and plaster and found a layer of blackjack on top of render on the stone.  I've been told to leave the blackjack in place and vandex over the top by two builders, but one says it must be rendered first the other says to vendex and then render.

 

Any thoughts as to which is the correct procedure?

[nod]

Unfortunately you will have to remove the blackjack before applying the slurry Sika is cheaper and a better product than Vendex