Hi, you say it is a rendered solid stone wall. Do you know what type of render it is? If it is cement render on solid stone wall that could be the cause of your damp problems and no amount of painting or fungicide treatment will cure the damp. The cement will have to be stripped off and replaced...
Hi,
Damp isn’t quite as straight forward as some damp proofing companies may suggest. Damp is often mis-diagnosed and treated with chemical injections and creams.
In order to offer advice I would need to know a little more about the building itself.
How old is the building?
What construction...