u/Biscoff22

We had a painter decorator paint our bargeboards - he sanded, filled and painted with two coats of Dulux Weathershield a month ago during a fairly cool period of weather. It's been quite hot the last couple of days and I've just noticed that the paint seems to be bubbling - difficult to see from the photo but it's not actually flaking off but definitely coming away from the wood. What's the most likely cause and is the painter at fault?