You should be able to skim over the wood just leaving filler in the cracks and such. Personally I don't think it is a good idea to use a lot of exterior spackle. I don't remember the brand but the exterior spackle in a yellow/gold [?] can is superior to most other exterior spackles. In the can it is off white instead of the standard white.

Why do you want to do a lot of patching on the exterior? You will have a longer lasting job if you can sand away most of the need for filler.