Re: Sealing a tempera work
I've sealed them with spray acrylic. Works well when the painting's dry, though if ET isn't perfectly dry, will develop "fog" beneath the acrylic, basically destroying the painting.
I've heard people have used damar to varnish ET with, but haven't tried it and can't be of help regarding that.
Iconographer's have long used a mix of linseed oil and stand oil to seal their ET icons. The process is rather lengthy, involved and the paint can come loose, ruining the picture. Suggest you do a websearch for "olifa" or alternatively "olipha" to learn more about this.
I'm no longer able to paint icons, but am more than happy to answer such questions as I can, if you contact me.