What are your thoughts on the product? What do you use for a first coat of sealer?

Haven't tried the Lenmar sealer . For oil based floors , I generally just use the same Lenmar OMU for all three coats .It works perfectly as a sealer.

I almost never use Shellac as a sealer because it is expensive, dries too quickly and melts when you try to buff it smooth.Although I do use it for certain floors when the need arises