Hi, Duncan,
Don't know if you've done the job yet. If not, what I would do is to paint a strong PVA adhesive mix over the sandstone. This will stabilize it and seal it ready for painting. PVA's available from any builders merchant or B&Q, as I'm sure you know. It dries colourless so won't show through any paint. I would give it a couple of coats. As far as the strength of the mix is concerned, I would go for 50/50 or even slightly stronger. Work it well in with a stiff brush on the first coat, then just a usual paint job with the second coat.
Try is....I think it'll do the job for you. Have fun!!
Denis (General builder and handyman)
Denis Langley, November 2007
