Most masonry coatings are water based, therefore, it is highly likely that the paint film had not had time to fully cure, before the shower of rain. Water based coatings dry mainly by evaporation. Drying can be slowed or even stop during and immediately after application of the paint by cold, damp or humid conditions. It is recommended that a solvent based masonry coating like Stormshield Pliolite Masonry Paint is used in lower temperatures or when rain is imminent.
High levels of moisture in the underlying surface and temperatures, 5ºC and below prevent the paint from drying properly, and therefore it should not normally be applied during these conditions or just before they are expected.