To fill older fastener holes that are no longer needed or useful, there are several methods.
If the undesired fastener hole is in a highly visible position, the simplest method to fill it is to use an oval head screw of appropriate size and a finishing washer. This disguises the hole into looking like it has a purpose. There are several such screws on my boat, serving only the purpose of filling an old hole.
Otherwise, a hole in a highly visible position must be filled and then top coated with color matching gel coat resin. To get a perfect color match and to blend the repair seamlessly into the surrounding gel coat area is difficult. If attempting this repair, I recommend first developing your technique on other holes in lower visibility locations.
For holes that are not prominently visible, you can fill the hole with tinted resin that is close to the hull color.
For holes below the water line, cosmetic appearance is not important. Just fill the holes with a waterproof resin.
For resin, I prefer to use epoxy resin mixed with appropriate fillers and tints. Epoxy resin produces a very strong repair and is waterproof. Epoxy also has a very high success rate of curing. Others use polyester resin to make repairs. Polyester resin will not cure if exposed to air, so special techniques must be used with it. Polyester resin is also not waterproof nor as strong as epoxy. The only disadvantage of epoxy is a tendency to yellow if exposed to UV radiation (sunlight). If epoxy is used in a high-visibility area exposed to sunlight, it must be top coating with gel coat resin. There is an old dock-talk story that epoxy cannot cure over polyester resin, but that is not true. For advice on using epoxy over polyester resin, listen to a podcast at
Classic Whaler Radio: PART 1 with Jim Watson of West Systemhttp://continuouswave.com/whaler/radio/ ... tPart1.mp3and
Classic Whaler Radio: PART 2 with Jim Watson of West Systemhttp://continuouswave.com/whaler/radio/ ... tPart2.mp3I have filled many holes on my boat with tinted epoxy resin and not top-coated them with gel coat resin. These holes are in low visibility locations or under the waterline. The slight mismatch in color and a general trend to age to a more yellow hue are only minor annoyances for these seldom seen holes, and the ease of working with epoxy is a big advantage for me. I have top-coated epoxy repairs with gel coat resin with good results.
The easiest method to fill an old fastener hole is to use some sort of caulk or perhaps a sealant-adhesive. I do not use that method, but it is a simple approach, and many report using it.
There are three good articles that discuss repairs; see
Repairing Hull Damage the Whaler Way
by Taylor Clarkhttp://continuouswave.com/whaler/reference/whalerRepair.htmlBoston Whaler Factory Instructionshttp://continuouswave.com/whaler/reference/whalerRepair.htmlThe Epoxy Cure
by Jim Heberthttp://continuouswave.com/maintenance-logs/epoxy/