Outdoor Finishes

This weeks question comes from Frank who wants to know more about outdoor finishes.  Here's his question:

I will be finishing white oak chairs, 1"  and 3/4" thick.  I have reviewed your website and book, and still have a question about wood finishing.  If choosing between an exterior Spar Urethane or Thompson's Water Seal to finish patio furniture, what would you use?  I have been to the Minwax website, and these seem to be the two contenders for outdoor furniture.  Minwax products are readily available.  Are they any good?

Here's my response:

If it were me, I'd stay away from the water seal products.  Here's why:  They don't last long enough because the layer of finish is too thin.  Most of the time these products are thinned down water base or oil based urethane finishes. 

The problem with spar varnish is that it  has no UV protection.   Water damages wood the most no doubt, but UV rays are a close second. 

The best way to protect your chairs for years of use is with a marine varnish.  You can find this at marinas or marine shops.  Here's the draw back with marine varnish.  It's glossy and it takes 5 to 7 coats at least to get the full UV protection.

So, with all that said and if you don't want to paint your chairs, I'd go with the spar varnish.  It's a lot cheaper than marine varnish and it leaves the wood still looking like wood and not plastic.  Just know that the oak will turn a silver gray without the UV protection.  To get the most life out of your chair I suggest that you don't sit them on the ground for long periods of time and bring them indoors during the wet winter months.

-Craig