I thought I'd pop in now that the decoupage job is over 5 years old and give an update...
The varnish is holding up well. There are a few scratches in the finish because we got a medium-sized dog in the interrim, and it took her a while to learn that the table is NOT her domain. None of the scratches have gone all the way through the varnish, they're just not attractive.
The bug is still there, I think he's probably good to go until some future scientist harvests him for whatever DNA they can find in the blood he was drinking shortly before he stuck to the table. "Homo Sapiens Park" anyone?
BIGGEST ISSUE WARNING: There are a few places where the decoupaged paper wasn't completely flat. The paper wrinkled a bit, I think I may have gotten it too wet during the gluing process and it buckled. At the time I thought I had it pretty well covered with the varnish. But, over time the ridges ended up poking out, and now crud has started building up under the corners of paper. Combined with the moisture from wiping it, I shudder to think what would happen if we cultured those areas for bacteria. We don't use this table to eat dinner any more but if we did we'd have to sand it down and start over.
So, if we were starting from scratch, I'd be very careful to make sure my paper was all very flat. The ridges didn't seem HUGE but what isn't a problem on an artsy fartsy project can definitely make a difference where a lot of water and wiping are involved.
Other than that, it's been amazing. A cheap, easy project for someone who really doesn't have any woodworking or painting skills. Just keep that paper FLAT or layer on a lot more varnish and I think probably anyone could manage this!