How can you paint over light blue paint?

First, lightly sand the poly to smooth out any bumps and to de-gloss the finish a bit. Use 120-180 grit sandpaper or sanding sponge. Or, you can apply a liquid sander instead of using sandpaper. This gives your primer something better to stick to. Next, apply a bonding primer - bonding primers are any primers made to stick to glossy surfaces (check the label). Oil- or water-based doesn't matter. Let dry. Then apply your finish paint in two separate coats (let the first coat dry before applying second coat).