External Sandex paint

[kfc06]

Hi I recently bought this paint: http://www.sandtex.co.uk/products/sandtex-smooth-masonry-paint/

Im painting onto a window sill that is made of terracotta tile. I used this paint thinking it was the correct stuff, but now unsure.

It was painted on a dry sunny day with plenty of drying time and all seems nice and even to finish. It then rains the next day and leaves a patchy result as though the paint isn't waterproof (although it says it is on the tin). This has happened twice now to me as at first I thought it may need a second coat or rained to soon after application but it appears not to be the issue.

Attached is an example of the issue. The darker part is the the lesser exposed part of the sill near the window, the lighter part is the outside edge of the sill.

Does anyone have any ideas why this may be happening? View media item 76908

 

[sparkwright]

Noticed this before with an outdoor doorstep paint.

Never got to the bottom of it. Was patchy for a long time, then then it all broke up in a hail storm.

Threw the rest of the tin away after that.

However, your paint is best suited to walls, so maybe a tile paint or similar may be the answer.