The function of Windex is to activate Monokote's iron-on adhesive without heat. You only use that when putting it on over film covering.

Iron on Monokote, not sticky back trim, can be adhered to film coverings using Windex to activate the adhesive, without bubbles. If you try to iron on Monokote over Monokote, the adhesive will gas, and create bubbles. Windex allows you to activate the adhesive without gassing. Squeegee it all out from under the Monokote, and it will adhere. Afterward, you can seal the very edges with a low heat trim iron, if you wish.

Good luck,
Dave Olson