When a hull is upside down what you see for the most part sits under water. The sides show. So it might look worse that it actually will be right side up. Is the weave filled? Or are you judging a glass job in the first stage? If it's not filled you can fine sand the bad spots a bit and sometimes they will wet out - assuming it's dry areas that caused the white spots.

I would just move on and varnish it and the first coat you can add tint if you want. Tinted varnish is not very opaque and will not cover the white spots. One thing you can try is an alcohol base marker from art stores. They have infinite shades of brown and you can touch up the white spots with that and blend it a bit with your finger. Varnish will stick to it fine. Or even some artists oil paint can do the same thing.