If you can see daylight through these cracks, then you should definitely fill them with epoxy thickened with wood flour. If these are just gaps from bad joints that still meet at the back and if you will be covering the area with fiberglass cloth, then you should be OK. The cloth and the epoxy should fill gaps of 1/8th inch or less. I think. :-)

I built a 14last year and had the same issue. I was able to fill it with wood flour thickened epoxy. I didn't like the final look so I applied auto pin stripe tape before clear coating.    

Did you try a putty knife or syringe?

Laszlo

   You can easily make a tool to help you with this. Cut out a huge plastic putty knife from an empty jug of some kind -- about 6" by 12". I used an empty 20lb kitty litter container. Put a smear of peanut butter epoxy about 3" wide on the tool. You can scrape this along the seam of your boat to apply a controlled amount of goop. Of course, it will only work where there is a sharp edge to scrape along.

   I did try a putty knife but found it difficult to squeeze the epoxy into the seam and avoid getting a lot on the side of the hull.  I already created some residue that will have to be sanded off and stopped before I made it worse. I also found that it was setting up too quickly to do any fine work.  I have syringes and thought I would try working with those but I’m not sure what mix to work with.  I see no daylite and the putty knife does not pass all the way  through.  BTW, in some sections near the stern the gap appears to be almost a 1/4 in but those are the exception.  Bruce

   You may find that turning the boat up on its side helps this operation since gravity would be assisting you. You also might apply the thickened epoxy as you would drywall mud. Yes you might need to sand it smooth. But good knife work will counter that.

I suspect the bevel of the joints just wasn't ..................... completed.

Either way you do not want any "air" voids behind the cloth.

i would fill the gap with woodflour thickened epoxy....mayonaise/whipped butter consistency.

when i do something like this, i use inexpensive blue masking tape (painters tape) on either side of the gap/seam that i am filling.  trowel it in... then use a spreader to push it into the seam and clean up the excess

then remove the blue painters tape and you will have almost nothing to clean up in terms of thickened epoxy sitting on the adjacent wood...potentially discoloring it.   this will also save you significant sanding time.

all the best, 

h