Can you vandex over blackjack?

[iamgroucho]

Evening all, I've removed a load of render from the inside of the front wall of my house and it's got a layer of blackjack on it. I want to coat in vandex but do I need to remove the black jack first or can you apply vandex over the black jack?

 

[23vc]

If it doesn’t say on the bag, call them and ask what options you’ve got