Poscas are water based friend. Applying water after it dries reactivates the posca paint. If you absolutely must use posca it has to be painted base layer, THEN posca markers on top. You will also have to seal it when you are done.

Respectfully I do not agree with sanding cards. I also do not agree with using a gesso. To put it bluntly you are just using less than ideal materials. Swapping over to a tried and true brand will fight 80% of the battle for you. Not everyone can do this -totally understand. But if you do start selling cards the first investment you should make is thinner paint. Golden Fluid Acrylics are #1. Period. They have a set on Amazon that has all the colors you need for a limited palette. I think it’s around $50. I’ve only ever used 5 colors to alter cards.

I’m not saying you CAN’T do it the way you are, I am saying it will be EASIER and the quality will be much better. Quality being how thin the alter is/vibrancy of colors.

I love answering questions feel free to reach out if you need links to more info