Step 4: Paint It
It takes at least two coats of paint to make cardboard to look halfway decent. After it dries, you can make it look even less like painted cardboard by brushing off the strange little fuzzy bits of paper crumbs that are dried on top. That smooths down the surface a bit.
I also painted a couple ancient circuit boards covered in microchips that I zip tied on to the frame after they had both dried.