When ever I spray a finish coat, The first coat I reduce with 25 % thinner. that allows the top coat to sink into the wood and displace the trapped air. That is what is causing your top coat to bubble. Your top coat dried too fast and trapped the air. Putting it in the sun made it worst. And if it persisted, add retarder. This thinned top coat is referred to as a piss coat in the trade
Will Williamson willcnc.com willmade.com
Williamson Lumber & Millwork Inc.