Is there a sealer to put on walls after they have been panted?

Answer 1

Don't know why you would want a sealer over paint.
If you have painted over mold or some stains, they will still bleed through new paint.

If that be the case, the best way to prevent this is to paint over stains and mold with aluminum paint, then apply the finish paint.
This works.

Answer 2

The paint IS the sealer. There should be no reason to put a sealer on top of paint. The only time I've seen a sealer used is if you want a glossy look over a Venetian plaster or other faux painting.