I tried using Rockhard filler on my first cedar kayak with very poor results. While it gets hard enough the color is very light. What I would suggest, if I understand your problem, is to cut some long taper strips and then route the cove and bead in them. You can get these very thin and tapered to the thickness you need. Trim to fit, glue and slide into place. You can stack a few if you need to. They look far better the filler.

: Some of the minwax fillers I've seen have been very soft and took a long time
: to harden...if ever. I really don't think that is what we are looking for
: in a filler for kayaks. There is a lot of searching for the
: "best" filler. I know some people have used RockHard wood
: filler. I have...but not for a kayak. I will try it for that some day.
: Most of the time I just use wood flour mixed with epoxy (I precoat the
: wood to avoid color blotches). If you haven't fiberglassed your hull or
: deck then you are not in too bad of shape for the fit. It is important to
: try to keep the deck and hull in the correct shape as your
: epoxy/fiberglass cures. This must be done while you do both sides. You can
: use spreaders, straps, forms, etc. to keep the proper shape and fit. Even
: then you may get a little movement, but it can usually be taken care of
: with straps, tape, etc. while you tape the deck and hull together.