what grit used to sand fibreglass

Just a caution, if the fiberglass has gelcoat on it try not to go through it. raw fiber glass will soak up any liquid and vapors and let paint peel. WEAR RESPIRATOR. Don't beath that stuff in, not even a little. If you want to Knock down high spots 60-80, smooth scratches 240-300. Good luck, Be Safe, you only get two lungs and need them both.