Air bubbles when Sandtex dries on roughcast wall

Hi, I've spent a few days painting smooth Sandtex onto a roughcast wall - what a truly awful task! Anyway, the wall was cleaned with anti-algae and thoroughly washed afterwards, then painted - this looks OK when it is applied, stippling into the horrible gaps etc, but when it has dried (with a single coat so far), it is covered with very small dimples which I assume are air bubbles in the paint that have risen to the outside while the Sandtex is drying. Any suggestions why? Should I avoid painting in hot weather? Should I thin the Sandtex more - or not thin at all? Am simply using a basic B&Q masonry brush, and being reasonably careful. How am i getting so much air trapped while painting? Many thanks.

 

Thanks. Not much I can do now, I guess, other than apply second coat when it's cooler? How important is a second coat, or would the one be enough? (even with the dried bubbles!) It's 'just' a rear garden wall, so can probably live with it needing to be re-done sooner than the claimed 15-year lifespan - anything is better than the prospect of painting it all again in a few days time, when cooler!