I refinished a table in my backyard. It came with my house, and was just raw wood and looked like hell. I was about to throw it out and decided to play with my sander. Low and behold, under all that sludge was a nice mahogany (or at least looked like it). After staining, I used Epifanes varnish:
https://www.westmarine.com/buy/epifanes--clear-high-gloss-varnish--P004_128_001_002?recordNum=3
I believe it is meant for sailboats, so my logic was that if it works on a sailboat it should be suited well for my back patio. So far, it has held up beautifully and I get compliments all the time about it. Its not cheap, but it sure looks purty and has held up great.