Teardrops n Tiny Travel Trailers

Hi all,,,,,been hibernating thru the winter,,,but just had my son in law spray about four coats of spar urethane over the two brushed coats I put on. Looks pretty good but want to know(if after it drying for a few weeks) can I buff it out by hand or with a buffer to really get a good finish? Walls and ceiling are natural birch ply....
Thanks TC

TC,

I know you can use a polishing compound on (very hard) polyurethane, but I don't know if it will work on (much softer) spar varnish?

For future projects, the best way to get a very clear, deep glass like finish with spar or poly is to sand/scuff between each coat.
The perfection you achieve will show in the next coat...

Rob