Sandtex reacting to Polyfilla

How odd. Even when the filler is not quite dry, a water based paint shouldn't bubble. A solvent paint might well do so

Further investigation yields that the bubbles actually contain water. I think what's happened is that we painted before the filler was entirely dry and the subsequent bright sunshine has caused the water to come out and bubble up the paint. A good excuse to wait until dry weather before doing any more painting.

Same thing happend here too. For some reason the Sandtex didn't like adhering to the Polyfilla. This was on an external concrete window sill that I'd filled some cracks in. Lasted only a few months before it began lifting!

I wouldn't bank on that. I mad damn sure my Polyfilla was bone dry before overpainting it with Sandtex but still got the same problem!

Presumably a bog standard water based masonry paint? These paints often contain small amounts of coalescing solvents to aid film formation. If these get trapped in the dried film, they eventually find their way out. Rather points to a Sandtex c*ck-up.

How would that account for the fact that Huge and I both experienced the same problem only over areas which we'd Pollyfilla-erd?

Filler seals the surface, so probably none of the liquid phase of the paint has penetrated into the masonry. If too much paint is applied in one go, possibly the film forms before the underlying solvents have evaporated. All supposition of course but "trapped solvents" is a well known hazard in surface coatings. Bad formulation on Sandtex's part

Or ours, painting in bright sunlight. The wall faces SW and gets warm to the touch on sunny days.