I am painting a wooden table for outdoor use. Once I've painted it, what coating, substance, or product do I put on the paint once it's dry to protect the table from the elements?

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If you already have at least a primer, and two coats of an excellent quality exterior paint on the furniture, then you really don't need to put anything else on it other than the paint. I would recommend at least a semi-gloss or a gloss paint on outdoor furniture as that will protect the paint as well as the furniture much better. You also need to follow all of the directions for application on the paint can label as to the surface preparation, application, drying time between coats, etc.