Marine grade mostly means its resistant to water damage and isn't as potentially porus as a regular poly. It's also UV stabilised, which isn't much of an issue for a guitar, but I like that it's really durable. I'm considering making it my primary finish option as it's pretty straight forward to get a nice finish and way less mucking around than cutting and polishing up a gloss. It also feel really nice in your hands, which I think is important for a guitar. Worth grain filling prior to application with a sanding sealer. I've just been doing an application, blocking back and then doing around another 2 or 3 applications and it comes up pretty well. I think using a sanding sealer will reduce the amount of poly I need to spray though. Will find out on my next build.