Can you poly a painted surface?

While I’ve never heard of this being done, I did it myself a few years ago with some success.
I made some wooden flowers for my wife to put in a planter outdoors. She did some nice painted decoration, and we didn’t want the weather to destroy them so we topcoated with exterior poly. Worked fine, and four years later they look good as new

Yes you can do this. I would lightly sand the bookcase after the paint has cured with 320 grit to both remove any bubbles or trapped dust particles and roughen up the surface of the paint itself. Poly bonds though mechanical adhesion so it needs a roughened surface to latch on to. Then lightly sand between coats of poly with 320 as well until you get the finish that you want for the bookcase.

-- With God's help all things are possible- even woodworking. Woodworking is not just a hobby, it is an (expletive deleted) expensive hobby.

Like Scott said poly should work.

But never use lacquer over enamel, because you’ll get an alligator finish.

-- -** You are never to old to set another goal or to dream a new dream ****************** Dick, & Barb Cain, Hibbing, MN. http://www.woodcarvingillustrated.com/gallery/member.php?uid=3627&protype=1

Thank you gentlemen. I appreciate the information.

-- Semper Fi, Randy Sr.

Just keep in mind that poly will yellow the finish. Since part of it will be white you may want to use a water based finish.