That fillet is there to make a smooth transition between the bottom of the coaming ring and the outside of the spacer stack. If it wasn't there, you'd have an abrupt transition which would act as a stress concentrator. The coaming ring could break off the spacer stack if the fillet wasn't there.

It should be smooth, large enough to make a nice transition but still leave enough of a lip for a spray skirt to grip.

It's also sort of difficult to put on and then sand if you glue everything to the boat and then try to apply it. When I made the coaming for my WD12 I built it up on a sheet of polyethylene so that it wouldn't stick to the boat.

Then I removed it to give it the final shaping and sanding, including the fillet. To make the fillet, I used my gloved little finger to get the size right. As soon as the fillet was on, but while it was still soft, I painted it with unthickened epoxy to smooth it. That way I didn't have to sand it.

It's a lot easier this way, with no interference from the hull, no trying to get sanding tools into the gap without scratching the deck up.

Have fun,

Laszlo

Thank you, Lazlo.  This helps.  I know exactly what you mean about scratching up the deck if you glue the coaming in place before filleting.  I tried a small section and ended up with fillet material on my nicely epoxied deck.  Your suggestion to use plastic sheeting around the coaming makes perfect sense.  Knowing that I am making a fairly substantial amount of fillet, the hint on smoothing with unhardened epoxy also helps.  That would be one awkward sanding job otherwise.  I've got the boat looking almost ready for seats and hardware; I really don't want to mess up now.

BTW: here is the latest picture with my grandson using the router to roundover the coaming edge.  Handing the tool to a 12 year old took some courage on my part, but he did a great job.  Very proud grandpa.  

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