Well I really wanted to get a blue rectangular cooler to convert to a mash-tun but unfortunately I found a red one tucked away in my barn. I think I'm going to spray paint it so i can get that 110% efficiency that I keep reading about.
Won't work, the plastic has to be completely infiltrated with the blue pigment for it to absorb the microwaves that destroy the glucose spontanogenerase. You might try covering it with foil, some people report good results with this shielding method.
What were you storing in this cooler in the barn? Sounds like it might be best suited for lambic production.