Hi Mike I'm afraid you will have a heck of time trying to finish these 6X6 Doug Fir posts.......
The cracks you see are normal aging and drying of such a large piece of wood...and they will continue to move. Attempting to fill the cracks will simply result in the fill showing cracks over time, and much of it falling out :-(
Painting over stain is possible, but will be an ongoing uphill battle, even with the higher quality exterior grade primers available today. And for the tight spots (e.g. around rails, spirals, posts, etc) that you will have problems scraping/scrubbing in the future (like next year), the paint job will gradually take a deteriorated look to it.
Sorry to sound gloomy, but my luck with painting decks has not been too good.