Seal and insulate the base plaster

Hello, since I am building a new terrace I am going 20cm into the base plaster in height. Since the base plaster (painted with some color which has seen better days) will then lie in the gravel or frost protection, I want to paint Bitumen on the plaster and then install a dimpled membrane. My question would be whether this approach is correct .... I would like to thank you in advance for suggestions and solutions. Regards Frank

Bitumen is ok. What do you mean by Noppenbahn?

The old paint would probably have to be removed beforehand.

If it is solid, not necessarily. Only what flakes off.