This question may be too simple but I don't seem to find a good answer. How can you tell if a scratch should be filled with gel coat or only sanded out. Maybe I could ask the question this way instead: How deep can you sand ? I am working on my boat right now, I use wet #600 paper to remove superficial scratches but I have quite a few that are not deep enough to be filled with gel coat. I also noticed that when I sand a scratch on the top portion of the hull that is turquoise or (blue something) that creates a color mismatch, will that go away after a while ? I would really apreciate your feedbacks on this, my boat is a 2004 and in a couple of days it's going to be damn cold here.