I’ve made many molds and castings using these two releases. When you spray on the PVA, mist on the first coat and the successive coats should be light. If it beads up, you sprayed too much. Wash it off with water, let it dry and spray again.
I used an oil base sculpting clay to build up the clearance on my cowling. It can be shaped and smoothed out to the exact shape desired. Lay one layer of glass over the the clay and lapped over onto the cowling about an 1", let cure, remove and completely clean the clay from the back side, lightly sand and apply two layers of glass on the back side and lap onto the existing cowling about an inch.