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
Can be purchased at most building
supply stores. Home Depot - Lowes
About $12 a Gallon