Gel coat hairline crack

I have 3 very small hairline cracks in the gel coat. They are the width of a hair about 1 to 1.5 inches in length and dont look to be deep. These cracks sit below the water line of the boat. Should I fix these or leave them be? My boat stays trailer in the garaged when being store. I believe these cracks happened due to a light impact from wave action against a wooden dock support. If I were to fix them I would be dremeling them out, filling with gel coat and sanding it down to a buff. I have done it before but only on much larger cracks.

Fix them as you describe if they bother you. Otherwise, they are harmless.

well heck, i guess i look at things a little differently. i don't see them as harmless, especially under the waterline. the gel-coat is the water proofer, fiberglass resin is not waterproof and does absorb moisture. i think cracks under the waterline would need to be sealed at the least

You are correct that gel cost is the sealer but on a boat that is trailered the OP will never see a problem over the life of the boat. He probably would not even if it was stored in a marina in the water. I had my Ranger for almost 30 years. Over the time I owned it I got numerous chips and scratches on the bottom. Some received fairly quick attention and others did not. I am sure there were some under the bunk boards that I never saw. Never had any issues with the fiberglass at all.

Most hairline cracks are through the clear gelcoat. There is a colored gelcoat below it that will act as a waterproofer Unless you see black, or red, or woven roving, you should be fine.

My bad - I missed the below the waterline part. Yes, they need to be repaired then.

IF THE CRACKS ARE IN THE "ON PAD" OR "RUNNING SURFACE" YOU MAY WANT TO FIX. SOUNDS LIKE THEY ARE LIGHT FLEX CRACKS WHICH SHOULD ONLY BE IN THE GEL COAT SURFACE. YOU CAN GRIND ALL OF THE GEL COAT OFF THE BOTTOM OF A BOAT AND WATER WILL NOT AFFECT OR SOFTEN THE HARDNESS OF FIBERGLASS. I HAVE FIXED A LOT OF HOLDING TANKS THAT HAD NO GEL COAT. PLEASE FEEL FREE TO EMAIL ME PICS . jfiberglass@sbcglobal.net

Thanks all. I went ahead and took care of them, they weren't deep. But I feel better knowing they are repaired.