Simple answer
Sand, fill the grain, sand, spray with glossy lacquer, repeat if necessary.
Another way would be to wet sand with a oil finish. This will fill the grain. Then squeegee off the "mud". Repeat if necessary.
Grizzly is near me and the last time I was there I picked one up so I have parts to look at.
Here are a couple of things I noticed.
The plywood used on the grizzly ukes has a very thin top layer, be careful not to sand through.
The heal on the neck is not square to the body resulting the the head being way to far back. This will most likely cause buzzing around the 12th fret.
Chuck the strings.