When you find one, let us know :-).
Seriously, I don't think the "perfect" exterior finish exists. None last for more than a few years and most only 1-2 years. If you believe the statement that all finishes are non-toxic when cured, you could try marine epoxy over shellac (SealCoat).
If you're keeping the birdhouses, my suggestion is pure tung oil, reapplied every year. See:
There is nothing that quite works as well as good ole "exterior" grade paint. Any thing clear will have to be reapplied every 2~4 years.
Thompson's is just paraffin and mineral spirits. A waste of money, IMO.
Yep. If you're going to use TWS, you might as well take your money and stuff inside the can.
I am constantly getting asked to pressure wash and "re-seal" decks. And people ask about what the best "sealer" is. I always tell them, a high quality exterior paint.
I also tell them the worst thing they can do to a deck is pressure wash it. It forces moisture into the wood, raises the grain and makes it all fuzzy and splintery. The best thing to clean a wood deck is a simple mold-killing house wash with a light rinse from a hose or pressure wash on a very low setting.
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