DIY Doctor UK DIY Forums

Hi all hope someone can help. We hired a decorator to paint the exterior of the house earlier this year. It's pebble dash. With some new pebble dashing after an extension and moving doors/ windows. Within a few weeks of the white paint being applied to the pebble dashing, the paint on the old areas of dashing yellowed. The paint over the new dashing has remained white. I don't know if the decorator applied any preparation before a first coat of diluted paint. There was nothing obvious on the old rendering that would stain. It's worse on the north side of the house and at the bottom of the wall, so I guess there was some algae or mould which has come through. It was a little blackened in places.

I now 'want to paint over to get rid the yellowing as I want to sell the house. I'm going to do this myself. Can anyone advise on what paint and treatment to use to stop the next coat staining too?

Thanks in advance for your help