Whats the deal on masonry paint, is any rain disastrous or can you get away with a light shower?

Using Sandtex ultra smooth which dries in 3-4hrs.

I had a bit of an epic one November trying to paint the exterior of a Teesdale house I was trying to sell in a hurry. I’d get a coat on, the rain would start and rinse most of it clean off. Took about a week of reapplying before it stopped raining long enough for it to dry.

Depends on the aspect of the wall – if the rain is being blown onto it, you may lose some coverage, although at this time of year, the warmth should help out and get it dried pretty quick.