For some strange reason I get blisters after a new coat of paint ?
I sand , prime and paint .
I checked on line maybe the summer heat makes this happen?
ANy painters on the board ?

Might be humidity thats causing it.

let primer dry?

On your house?
Is the stuff you are painting dry?

Did you use a pressure washer on the house before painting.

If you use a pressure washer to strip peeling paint and wash off dirt and mold , a lot of water gets forced into the siding/shingle etc and it takes a long time for that moisture to dry out before you can paint over it.

i once saw a guy pressure wash his house then paint over it the same day. the paint job looked beautiful until about 2 hours later when it all started to slide off like it was melting butter.