In November, we painted 2 rooms semi-gloss, over satin. We did not prime, not knowing we had to. Now it peels off super easy. It’s not peeling on its own, just if it’s nicked or scratched, but I am able to peel it off in chunks, like nail polish. We obviously need to fix this. So question is; Do we have to scrape off all the new paint, THEN prime? Or can we prime over the existing paint and repaint again, and not have this issue? We used Glidden.
Sometimes we ask questions hoping for a different answer than we know we are going to get. Sadly, yes you must remove the peeling paint.
After removing the peeling paint, yes all of it, you will need to sand the sheen of the existing semi-gloss. I have to wonder if the semi-gloss is an old oil based paint. In any case, visit your local paint dealer and purchase a bonding primer made for the purpose you require.
Prime, one coat should be sufficient, and then paint 2 coats. Simple as that.