Advice on clear coating a painted picnic table

If it will sit outside in the sun, nothing will last very long at all. I have a south-facing door that I have to refinish about every 2-3 years. The best thing I have found is spar urethane, but even that doesn’t last long enough.

a clear coat is not necessary over exterior paint. Go with a higher gloss as it will dull down quickly. If you are go with a clear get an acrylic clear for exterior, oil based will amber.

Some issues you may encounter:
A water-based product should adhere to the paint, but may not perform well in direct sunlight. You’d probably need to move it into a garage or at least a covered area when not in use. It will absolutely blush if exposed to standing water.
An oil-based product could possibly lift your latex coating, ruining the entire project. And as stated above, they are not water-white and will amber over time.
I’d suggest leaving it painted and touch it up as needed, or abandon the paint altogether and use a solid-color latex stain or deck finish. It will look like paint and is designed to handle the elements. Make sure you coat ALL sides of the wood, paying extra attention to the ends.

Here is what happens wood moves and the finish won’t move at the same rate so when the wood moves the finish won’t expand and contract at the same rate and the finish will sheer and fail eventually. The best thing yiu use on wood is a hard wax oil. What will help is put boiled linseed on the wood first then apply your finishes drier wood moves more. But the boils linseeed oil takes a while to dry cure like a while

There is no good reason, that I am aware of, to clear coat painted wood. Spend your money on a high quality exterior paint.