Using Bondcrete in exterior paint

My Mum is having the outside of her brick house painted and the painter is stumped trying to work out what my father (who is dead and so we can't ask him) did to make the existing paint stick so well.
My recollection is that he mixed Bondcrete into a waterbased paint (about 3:1 ratio of paint to Bondcrete) and used that as the paint. But maybe I've got confused.
Anyone have any ideas on this?

Bondcrete had (last time I bought a tin) the ratios for mixing it to paint or water as a sealer and surface prep on the side of the tin.

My father (also dec) recommended the use of bondcrete for cement and brick work.

I use it for pen gluing as it always remains a little flexible to absorb the wood movement without cracking.