Okoume is a hardwood (really) while the "cedars" aren't. The two species have markedly different characteristics despite they're both being well-suited to boat-building. One of those characteristics is porosity, another is grain strength.

Okoume can take sanding with a finer grit w/o clogging its pores markedly, will yield a smoother surface for epoxy. Before epoxy's applied, cedar generally won't benefit much from anything finer than 120 grit and in fact the coarser grit leaves a toothy surface well-suited to epoxy.

You know what kind of cedar your kit's strips are? Western Red's fairly common (Thuja Plicata @ 23 lbs/cu.ft), will be light cinnamon colored. Alaskan Yellow's not as common (not a true cedar either; Cupressus Nootkatensis @ 31 lbs/cu. ft) but makes a good choice, as will Nortern White Cedar (Thuja Occidentalis @ 22 lbs/cu. ft.) when you can get it. The latter is still the cedar of choice for planking traditional wooden canoes. Any of them will take sanding well with the coarser grit before applyng epoxy + fiberglass.

   As far as what type of cedar, I'd say "all of the above." There's a couple different darker species (western red and something else) and at least one of the lighter (looking at strips, not sure I can tell).

I guess I'm not following you regarding how well each wood will take sanding with a courser grit. My concern is around the combination of wood + sanding grit + epoxy/fiberglass. Are you saying that since okoume is open grain it will still provide a good mechanical bond with epoxy even if sanded with a finer grit (like 220)?

I've found it very difficult to prevent scratches in the cedar when sanding with courser grits. I try to sand with the grain but it seems like even 10-20 degrees off can show up if you're not paying close attention. In particular where my strips change direction near the bow and stern.

Thanks!

"...since okoume is open grain it will still provide a good mechanical bond with epoxy even if sanded with a finer grit (like 220)?"

Pretty much, yeah. Perhaps than manual you quoted is a little dated?

Both okoume and cedar get epoxied as a waterproofing layer, then some areas get a  skin of fiberglass cloth embedded in epoxy for toughness and added panel strength.

Where a 'bright' finish is intended, after final sanding varnish goes over both the epoxy-only parts (typically two or three coats) as well as those having an epoxy + fiberglass matrix: epoxy's protecting the wood, varnish protects the epoxy.

How these surfaces are sanded before being varnished determines how quickly you can build varnish to the smooth surface desired. In both cases it's the epoxy that's filling the wood grain; where there's fiberglass cloth, enough epoxy is added as filler coats to level out the texture of the cloth. For a bright finish, smoother is better, hence the finer grits.

My experience has been that those sanding scratches you see in your cedar will disappear once the surface has been epoxied unless you're using only 50 or 80 grit stuff. 100 or 120's enough to smooth the wood in preparation for epoxy.

On raw wood, successively finer abrasives are fine for varnish alone but with epoxy going down first 150 is as far as I'd want to go, if that; 100 or 120 ought to be good enough. 

And I forgot to add that stepping back three or four paces helps with the broader view.

When we're building these things, stuff that bothers us, that we see when close up, tends to disappear in the larger view once we step back a few feet.   

"......wood (cedar strip) should be sanded to 80 or 100 grit........"

Personnally I start at 60 grit and get the shape of a strip boat right then work my way up through 80 grit to grind out the 60 grit scratches, then 100(often 120 grit) to work out the 80 scratches,and 220 to get rid of the 100 grit scratches .  Okomee might take much less large grit use before going to the fine.   Either way you won't get the wood finish any better after glass. 

I have no problems getting a good bond to a smooth wood.  

  Thanks for your responses :)

My takeaway on this is that sanding to 120 should be fine on cedar strip. Sanding to 220, or maybe finer, should be fine on okoume and other open grain woods.

>I have no problems getting a good bond to a smooth wood.  

I guess it would be in interesting experiment to sand some cedar strips and some okoume to different, successively finer grits, then add epoxy & glass & see if there are any problems.