No Alligators Please

[Michael Dobony]

This would explain a layer dried in the sun, but doesn't explain why a second layer would instantly bubble up before being put in the sun.

[norminn]

The top layer formed a skin, then the solvents trapped in the earlier layer expanded but couldn't pass through the dry skin.

SOP is to paint each side of a house before/after the hottest sun exposure, for the same reason. Temp guidelines on paint labels are there for good reasons - too cold, it won't cure; too hot, the surface is complete but traps the lower solvents.

[Ed Pawlowski]

The solvents in the second layer softened the skin of the first and allowed the underlying solvents to move freely. The alligator you see is that softened skin shrinking in place under the second coat.