Many beekeepers use a combination of linseed oil and beeswax. If you do a google search I’m sure you can find a good recipe. Some have found that linseed oil by itself tends to mold more than tung oil, which is why I use tung oil on my own hives.
Whatever sealant you apply, only apply it to the outside. On the horizontal top bar hive I usually only apply it to the roof, leaving the hive body bare.
The follower boards are used to expand and contract the amount of space the bees have access to. You start them out with a little space, and then as the colony grows continue to add more.