we paint with mathews acrylic polyurethane, no primer necessary and actually not recommended unless it's an acrylic primer.
paints great and is our standard for use of sintra signage. especially for outdoor panels, as sunlight/weather turns uncoated sintra to a faded-white version of whatever color it came as and actualy makes it want to potato chip after a couple of seasons. i have signs that still look brand-new after years of wisconsin summer/winter's.
light sanding if you're paranoid there isn't enough "tooth", but the edges always have a rough-cut anyways so it's worthwhile giving it a solid rubdown with scotchbrite prior to painting.
don't try to over-paint it the first go-around. give it a solid dusting to begin your coverage, let dry for a few min, and then go back for a second coat. repeat for a third if you've still got some light spots, but just don't get carried-away or your panel will take forever to dry and may outgas & create bubbles from under your vinyl if applied too soon. I usually wait at least a day to dry, before I dare apply vinyl graphics.
