I built 2 sets of shutters for the window above the garage door, one set for my house and one for the neighbor.  The incredibly crappy shutters installed by the builder lasted 3 1/2 years for him and began rotting at hte bottom of the panels.  Mine are only cracked at this point. 

The frames are 5/4 poplar and 3/4 for the raised panels.  In the interest of preventing future rot, I (a) primed every piece before assembly with diluted acrylic primer with a dose of M1 mildew/mold preventer.  (b) The end grain got a second coat, allowed to soak in well.  after assembly, both sets were sprayed with a light coat of primer.

(c) To prevent rainwater from being trapped in the dadoes at the bottom of the panels, I drilled weep holes from the dadoes to the back. 

I sprayed both sets, mine with SW ext. trim paint used on the rest of the house.  He chose SW Emerald polyurethane trim enamel. 

Both sets were dried in the shade for several days.  Then I moved them and the sun hit them for a couple of hours,  and 3 of the 4 panels on his bubbled severely.  None of mine bubbled.   All the panels were cut from the same 12′ board, so its not a wood issue; MC was 8-9%. 

I’m thinking that the PU, which has great cohesive strength and less adhesive strength, blocks any gas/moisture from escaping, while the regular acrylic latex does not.   He returns from vacation next week and I want to have a solution.  

at this point, my thinking is to remove the PU and have him get some acrylic latex.

Anyone have any thoughts?