Painted Birdhouses

I had a few boxes and they are painted with acrylic paint. I want to put them out but I think I should put something on them to keep the paint on and to protect from rain. What can I use?

My son and his friends build houses for living. they use spar varnish, but be outside the varnish, as porch paint.
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The person in the painting department at the local paint store is the expert - I would suggest a clear high gloss lacquer that is designed for outdoor use. Make sure it does not turn yellow or opaque to sunlight.

If nest boxes hanging by a thread, you can also put a squirrel guard about 6 "-8" on them - it keeps the squirrels on and also provides pretty good protection from rain and sun.

Clear acrylic sealant ... aerosol preferable ...