Hi! I'm trying to screw a piece of aluminum to a section of plywood, and I want to make it perfectly flat against the plywood. However, no matter how careful I am to smooth the aluminum, by the time I get to the final screws there are several portions of the aluminum that are annoyingly separated from the wood. When I did this on the second piece of plywood, I was even more meticulous in affixing the aluminum but I got the same result. Tomorrow I have to do three more pieces. Any tips on how I can do it so that the aluminum is perfectly flat against the plywood in all places?