Sealing sandstone before painting (exterior)

Please don't take this as off topic but no paint stops rising damp problem. It needs to be addressed before any painting as the results would be only temporary.  Most of paint manufacturers these days making ,,self priming" paint. It works saving you a buck for the primer. However special primer is always recommended for the troubled walls and there's a plenty on the market, though I used some sealant without much of difference on the results on my house walls. Quality and durability was the same on untreated walls and been like this now for a quite few years.  

Sandstone and stone paint is a separate category though many masonry paint would cover this and normally if you read the label, there's always manufactures recommendation to use they primer with this product. As mentioned above, there's special stone primer available but it could be quite expensive as others general stone and masonry sealants and primers. You might need some translator and check a few labels in the paint shop.

Hi natural stone doesn't like most paints as it needs to breath to stay durable, as another has said. Firstly there are two types of damp, rising and lateral, to eradicate rising damp an impermeable barrier is required  preferably just below ground floor. And ideally a min 150mm above path or garden level, this can be done by inserting a physical dpc membrane or a chemically injected one which is applied under pressure or hydrophobicly. Lateral damp however is where a surface coating can help. Using a microporous coating coloured or clear will allow the stone to breath, letting damp escape whilst at the same time repelling rain water.Remember stone contains quite a lot of natural salt which tend to be mobilized by dampness and migrates to warm surfaces during the drying out process. These salts will eventually blow away in the wind as the stone dries out. Do not try to wash salts off as it just triggers the migratation cycle of again.

Kiem Granital Mineral based paints have a full range of colours and carry a long UV stable life span last time I looked it was 10- 15 years.

Hope this helps 

Hiya, that's great, thank you. We've had all the house internal walls treated and damp proofed in January and the house needs to dry out, so we're just getting to grips with what to use outside so we don't push the damp up, so to speak, as it won't penetrate back in due to the damp proof course.  In, out, shake it all about .....!!  Good advice re the sandstone primer, once again thanks. 

Alan , thanks for the comprehensive answer - very informative.  I've just replied to another response on this thread as we've had the damp proof work done in January.  I know re the salt like residue - what a pain/mess!  I think I know what to do now re the sandstone patches.  Again, thanks for taking the time to respond.