I'd use 5 minute epoxy. I mix it with some coffee grounds to get a nice dark filling for the knotholes. Sawdust would work too, but may look unnatural in a knothole. The good thing about epoxy is that it won't creep like PVA glue and it's much harder.
Sanding is definitely a pain. A block plane or scraper works quite well on epoxy. Just make sure you let it cure for 24 hours. It hardens within 5, but it will still be a little gummy on the inside.Epoxy will break down from exposure to light over time if there is no top coat of finish over it, but it doesn't require much to protect it. Just a little lacquer or varnish. Get it very near flat when you fill. A little "fudge factor" that seems like a good idea when you are filling it is a major pain to sand away later (DAMHIKT).
Cocoa powder makes for a terrific medium brown. Not kidding. I haven't gotten into using Paprika yet, or Curry, but I'm sure one day I'll need a special color....Coffee Grounds! Why not! Now there's an idea...
My main complaint with some of the epoxies from the big box stores is that it's hard to measure them out for the exact 1:1 mixture (eyeball it?). The smaller tubes with the self mixing nozzle/head work well, but you only get two (2) nozzles which can't be cleaned (supposedly acetone or ethanol will work if used right away). For small projects like filling knots you'll have a bunch of leftover epoxy resin and hardener in the dual syringe tubes, but no nozzles to dispense and mix it . Cost effective? :icon_scra
The chemical reaction which creates epoxy only happens when the resin and hardener are mixed so the commonly held theory that either component "gets old" sitting on the shelf is hogwash. Store it as recommended by the manufacturer and it should last for years.
West System epoxy is a little pricier, but you've got a lot of options from resins, hardeners, open time, cure time, fillers, etc. For starters the #105 resin (1 qt) and #205 hardener (1 pt) is an economical choice that'll give you a lot of epoxy for your buck. The metering pumps ($13 for the set) are designed to dispense the precise amounts of resin/hardener each time you use them. No need to clean them either, but they can be cleaned and stored away if you wish. Check out the West website for loads of useful information on product selection and applications.
http://www.westsystem.com/ss/
ccasion1
The chemical reaction which creates epoxy only happens when the resin and hardener are mixed so the commonly held theory that either component "gets old" sitting on the shelf is hogwash. Store it as recommended by the manufacturer and it should last for years.
I absolutely agree about the resin with one big if - if it is 100% epoxy resins (a little inert filler is okay), but I respectfully disagree about being able to make such a broad statement about hardeners. I have had some hardeners (RAKA medium is an example) that do seem to last and last and others, WEST being one, that don't keep as well. There seems to be some volatile stuff in the mix and I am not sure if the hardener breaks down or if something evaporates or condenses enough to throw the ratio off, but the better hardeners that give you the hard clear surface seem to have a shorter shelf life.