I built a skerry kit about 5 years ago, and don't recall any specific percentage called out in the manual of filler to epoxy.  I recall it saying it should be a "mustard" consistency or slightly thinner, which makes sense for using the syringes.  One thing is that the kits have been shipping for that long anyway with Cello-o-fill instead of the colloidal silica that CLC originally did the prototypes with and wrote manuals.  I know from the canoe I did 15 years ago and the skerry that the silica and cello-fill do mix differently, so go with the consistency, not a specific ratio.  I probably went on the thin side, and I had to go back a couple of times and wipe up the drips that made it through the seam, but at least I could pump the stuff.

As for mixing, I always stir the resin and hardener a lot before adding filler, and add the filler in three or four steps, partly because I'm looking for the right viscosity, partly because the filler can clump up if too much goes in the epoxy at one shot.

BTW, West System sells a high density filler, 404, that is a mix of silica and calcium metasilicate mineral dust in addition to their straight 406 silica.  The 404 mixes quite differently than the Mas products that CLC ships, is very strong, heavy and crazy hard.  You never want to sand that s***.  Overkill for nearly any CLC project.  I got a can by error for a big boat project.  Thank heaven it was hidden in the bilge and no sanding.

   Another thought, if it is chilly, like in my basement right now, the Mas resin gets rather viscous and if you are using the metering pumps, you need to be patient and let that plunger make its way all the way back up so it fills completely.  Otherwise, you might wind up a bit short on resin (long on hardener) which might make for erratic cure times in the pot.  

I'm working on a Kaholo right now and I've been saving up plastic food containers (yogurt cups, deli containers, takeout chinese) and mixing in those so I can toss them after a round without feeling guilty.  I keep a couple of quart paint cups for big batches and I wipe them out for reuse, but that's all.  So, I've been mixing everything from neat epoxy to filleting mix in a range of cup sizes, and have had 100% successful cures and no pots have cooked off.  I like Mas.  I had a West System batch cook off a while back and it was a good thing it was in the garage so I could toss it outside.  Mini volcano.

When I was tacking the seams on my build last year I took the plunger out of the syringe and stuffed the epoxy into the wide opening. Then re-inserted the plunger and squeezed. Lots easier than trying to fill the syringe through the nozzle. It's a bit messier, but that's why I buy disposable gloves.

Like Mummichog, I use Cell-O-Fill to thicken the epoxy to a mustard-like consistency. It's colloidal cellulose - the stuff baby diapers are made of - so you don't need to wear a respirator while using it. And it sands like a dream after the epoxy hardens.

Thank you all. Starting with mixed hardener and resin, and then adding cel-o-fil "to taste" seems like the way to go, I'll try that today. I had been starting with cel-o-fil, and then adding resin because that seemed to allow for more precise measurements. Maybe that is why I experienced the clumping yesterday.

About the pump plunger: Yes, I am careful to let the pumps return all the way up before pressing them again.

I also built a Skerry 5 years ago, then a Chesapeake 17 LT then a Kaholo and lastly a WD 14 and sailrig, as you do.

For all the seams I make up the epoxy mix, dump it all into a cake makers polythene piping bag, snip the pointed end off respective to the size of fillet I want and squeeze the mix out. Piping bags are dirt cheap and readily available. 

Much better now. I mixed up hardener and resin, and then mixed in cel-o-fil slowly, to a manageable consistency, and things are moving along very nicely now. Thanks again.

   So, I just finished building an Ultralight.  All I did was tack weld with CA glue, remove the wire and then epox fill the seams.  No syringe no cake mixer.   Just a wood mixing stick to spread and a rubber squeegee to remove the excess.  The boat came out beautiful.  Don't over think it.

   A little aside on this subject - after tackwelding with CA, I've had success in limiting the obnoxious and time consuming effort of chiseling/planing/scraping/sanding any epoxy seam leak-thru drips by putting a line of standard clear box-sealing tape on the opposite side of the seam, wherever possible and wherever the risk of a leak-thru was obviously likely, and pressing it down well into the crack.  Always take care with a light sanding or alcohol wipe to make sure no tape residue remains, but if tape is pulled as soon as epoxy is firm, I don't think residue is an issue.  Putting tape on takes much less time than chasing wet leak-thru drips and sanding hardened ones.

Bags are good, I've used those.  Also used a syringe to good effect.  Perhaps my wife's biggest contribution to my NE dory build was filling one syringe while I applied the other to the inside lap seams, then I'd swap her a syringe needing a refill for a full one.  Doing the inside seams (with the boat rolled to an angle to create the correct "leveling" of the epoxy resulted in a very nice, small bead (or filet, if you want to call it that) of lightly thickened (thus almost clear) epoxy that made for a very nice inside finish, and helped avoid sanding down the lapped plank edges too much while finishing.