Paintwork bubbles

Heat bubbles this could be . You tend to find that as your door step was outside and could be in direct sunlight whilst taking time to dry the heat from the stone while cause this reaction to happen

Paint bubbles can form long after the paint has dried. Take steps to avoid this problem before it occurs. Paint blisters or bubbles occur when the paint film lifts from the underlying surface. The loss of adhesion between the paint film and surface is usually caused by heat, moisture or a combination of both.