Frank, I agree with Chris on this. Use epoxy filled with wood flour or you could even use wheat flour if color match isn't important.
If you have through holes to patch like where the knot was in that photo, cover the opposite side with plastic, i.e. packing tape, polyethylene, or even part of a blister pack type of packaging. The epoxy will relase from the plastic when it cures and leave you with a smooth surface.
On blind holes, fill with thickened epoxy, cover with plastic and press it flat. Leave it overnight and voila! No sanding required.
If there are small holes that thickened epoxy won't fill easily, just run straight epoxy in.
Epoxy will stand up to exterior applications however it has no UV blocking ability. For filling these holes it would be the perfect material. Epoxy is actually very easy to use. Just don't mix large quantities of it at a time. If you do you'll find it gets quite warm. You can use if straight (just resin and hardener) or thickened to any consistency you need such as mayonnais or peanut butter.
Raka is a good source. I buy my epoxy from them. I like the mix ratio and have never had a problem with it. They offer a couple of different hardners. Fast is good in low temp situations but you'll definitely want the slow stuff in the summer.
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