I was unwise enough to try painting the chimney stack over the cold spell and the cold , combined with the rain has left a misty streak of white paint wash down the slates.
I have tried to wash this off with WD40 as well as thinners but it doesn't seem to work.
Should I put my hope in the weather to wash it away over the coming months (seeing as how exterior paint is supposed to actually be water soluble)?
Or will this be a permanent feature unless I do something now?
What about a wire brush?
I suppose the area is about 5 foot wide by 10 or 12 feet down to the eaves.

I did post an earlier thread on this forum and on the subject at
removing exterior paint from slates
but as I said I didn't really have any real success with the suggestions and have even managed to make the situation worse in the meantime....

Haven't read the others but have you tried Nitromorse.

Check LTP ONLINE

Tj has it right

nitromors or its equivelent

biostrip is good