CNC Router American Flag with Liberty Quote

I found this image via Pinterest (https://momsaysitscool.com/products/9880557) and knew right away it was something my brother would love so I Googled for techniques and settled on the following (based on post at How to paint V carve clean):

Lightly sand and clean a piece of 1/2" plywood prior to applying a layer of polyurethane. Allow to dry overnight. Pour a generous coat of rubber cement over the top and spread around with an old giftcard, leaving behind only what couldn't be scraped off w