Sometimes you will get air leaking out from under the bait if its not seated well.
As pop dries it shrinks away from the bait. I pour my second layer not long after the first layer sets up and I don't use a heavy coating material. I simply use veg oil and let it soak in. I also don't coat the bait with the oil.
Another tip would be to not use as much water in the second pour. Use just enough to allow detail transfer. ( the coating not only stops the pop from sticking together but it also repels water and that can cause voids.)
www.novalures.com
