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Given that this is a Seiko Quartz brand I highly suspect the face should be a mineral crystal. The best way to check is to tap it and listen if you have a high frequency most likely it is a crystal, if it has a dull sound it's plastic. You can also check by its reflection. A plastic will have a warped reflection where as a crystal will have a non-distorted reflection. Regarding the finishing, if it is a mineral glass you it would be best to remove the crystal from the watch or mask off the steel area. I usually use a 1000 grit wet sand paper and slowly grind away the crystal until all the deep grooves have been removed. It takes patience however it does payoff dearly if you can't find an new old stock replacement. I finally finish it off with Cerium Oxide (glass polish). Again, takes a lot of time and muscle power, however you are paid off with a perfect crystal. Make sure the crystal is flat on the wet sand paper, you don't want to start beveling the sharp edges. Removing the crystal from the case could also help with the case restoration work as well.
Nice watch and a great find!
Good Luck!
Regards,
Robert