Around 4 years ago, we painted a chalkboard for our 1-year-old kid. She loved it and did all sorts of artwork on it. Now she is 5 years old and doesn't want the chalkboard anymore in her room.
I tried to scrape it off using sandpaper and a Dremel power tool (MM 50) to powersand it. 
On a suggestion from a home improvement store, I painted the Killz Latex based Primer over the blackboard. I cleaned the blackboard with a wet rag dipped in Dish Detergent and water, before applying the primer. I let the primer dry overnight. Now it looks like this.
I am planning to paint the entire wall with VALSPAR https://www.valspar.com/en/products/interior-paints-primers/signature-interior-paint-primer
Should I do a few more coats of the primer before applying the paint? How can I hide the blackboard completely?
Best Answer
I would give it a good sanding with 100 grit paper and then 220 grit paper.
I would then give it two coats of a shellac based primer, it is very good at covering or hiding dark colors and stains that may otherwise bleed through other primers and paints. It is a very thin product and you may think that is not hiding the black board, but just because you can still see some black does not mean that it is not providing a sealed coating.
I would then give it coat of a good bonding primer, these are thicker and formulated to bond well to stubborn surfaces.
Now you have a nice primed surface that you can paint with any type of paint you wish.