Minwax is actually a brand name for a wipe on polyurethane varnish. It is oil based and I normally use a local equivalent but use the term Minwax because I use a variety of different brands available easily and locally.
The sealing is to provide a barrier to moisture and even later paint solvents from attacking the structural bonds within the paper, the paper to foam bonding an finally the underlying foam layer itself. If not sealed the paper can absorb water from the air and soften or even come loose from the foam spoiling the appearance and even the performance.
You can skip the sealing if you use water based paints of cheap rattle can paints using many light coats with much time to allow them to dry completely between coats but I use the Minwax sealing to allow me to apply single and rather heavy coats in a single application, (I get a little impatient to see it airborne!
The subject plane is still flying and so far the only real incident was allowing a student have a fly of it and she stalled on landing and broke a prop, (no other damage so far in around 70 odd flights).
Have fun!