Any wood preservative would have copper
or something that might harm seedlings. I have used treated wood for raised beds
with no problems. But not for seedlings.
It is a rubberized roof coating that
would work for a couple years. Put two coats and let dry for a couple weeks. Of
course you need warm temps for any type coating to dry properly.
Can be purchased at most building supply
stores. Home Depot - Lowes
About $12 a Gallon