Paint or gel coat
Hi Sylvain -Good for you for tackling those pesky cracks! Sounds like you have a good plan with the Dremel. You could either fill with epoxy/filler and paint, or fill with gelcoat and wetsand to a fine finish. The trick is getting a good match. I know of a company called GCP (I'm sure there are many more across the country) in Seattle that will color match gelcoat to your boat if you provide them with a sample (cockpit seat, anchor locker, swimstep shower door, etc). It's best to try to find a smallish item to remove in the same area or at least same side of the boat in order to match the same color that has invariably changed somewhat over the years due to the effects of UV rays. Even if it's not a perfect match, it'll look a lot better. Alternatively, paint the entire deck and enjoy a sparkling new looking boat (that's what I did on a 1976 Islander 28 years ago). It all depends on how big a project you want it to be!Best,Trevor