Thanks Mark, for the tip about sign painter's varnish AND for the tip about the church in Solon, not so far from here. I love making Sunday trips to beautiful churches with nice icons; this gives me something to look forward to.
Would exterior polyrurethene really be so ineffective if, as I said, there was some protection from the weather? Just asking; I've never done anything for exterior use but have been considering it, though certainly not on the scale Fr Deacon John is talking about [4' x 5' !]