Can you put Polyurethane over painted furniture?

Certainly depending on what paint you used you do need to to seal the paint

Absolutely you can. I always do it. I have done it over regular paints, chalk paints, and stains and it looks great on all. I have not tried metallic but I would think it would look and do fantastic!! It requires mulitiple coats. Because of bonding issues you either have to get them all done within 1 and half hour times ( I try 45 mins) or you end up waiting 24 hours. I do fewer coats if I am not worried about damage. If you already know this--sorry ignore lol

Careful, it may cause a crackling effect. It is best to use water based poly on water based paint and oil based on oil based paints. Keep in mind that oil based may "yellow" the paint. Test on a small area.

If nothing else, get a piece of scrap wood, paint it with your various paint combinations and put the polyurethane on in as an experiment. You can also see how many coats it will take to get the look you want. I would try to get a clear one though, so that your colors aren't changed.

Make light colors with a dark coat.

It works well. Be careful with pale colors though - I've had it yellow white and pale pink resulting in redoing that part.

Yes, I prefer the crystal clear satin finish myself. Watch out you don't get one that will yellow.

Yes, Procrylic is water based and clear. Polyurethane has an amber tint to it.

yes just be sure it states it will not yellow

You can but I would use a Polycrylic instead, sometimes the Polyeurathane yellows. For surfaces that get a lot of use, I use Gator Hide from DixieBelle, it's awesome and it's safe to use and water repellent

Thanks for the help. I really appreciate it.