What should be placed between a plywood patch and the pressure plate to prevent sticking if glue is extruded from the joint?

Placing a piece of waxed paper or vinyl plastic between a plywood patch and the pressure plate is the correct approach to prevent sticking if glue extrudes from the joint. Waxed paper or vinyl plastic are non-porous materials that create a barrier, preventing the adhesive from adhering to the pressure plate once it has cured. This method also allows for easy removal of the pressure plate after the adhesive has set, as it eliminates the risk of the glue bonding the two surfaces together.

Using aluminum foil, another piece of plywood, or a thin metal sheet would not effectively achieve the same results. Aluminum foil may not provide the necessary non-stick characteristics and could potentially crinkle or tear under pressure, leading to uneven adhesive distribution. A second plywood layer could actually promote adhesion, creating a bond that could be difficult to break after curing. A thin metal sheet might not be flexible enough to ensure complete coverage and could also risk scratching or damaging the plywood during removal. Thus, waxed paper or vinyl plastic is the most suitable choice for this application.