After 3-4 hours of drying, normal rain will not affect the finish due to immediate calcification. Do not apply Classico Limewash on the exterior if there is a risk of heavy rainfall during the 12-hour drying time needed for the product to bond and harden correctly.
If it has rained prior to application that is okay. You should dampen the brick before applying it anyways. As long as the rain is over for a while (see above), you can get started.