There "used to be" a product called "Joli Glaze" that worked great for decoupaging and it help up pretty well on outdoor projects such as what Seedsower has made (which is very nice btw !). I did a similar piece (but not as nice) for someone back in the '70's using the Joli Glaze and it held up very well for over 10 years.
I can't seem to find a source for it anymore. Michael's carried it for a while but it's really hard to find now.
Maybe a ceramics supply place would have it or something similar that doesn't need firing ???
I've tried the marine varnish but it yellows after a while and ruins the colors underneath and tends to "fracture" the paper underneath.
That may be due to the summer heat here, though, or too much direct sunlight.
For all I know I might be spelling it (Joli Glaze) wrong. ???