Now that the situtation is more clearly described, I'd avoid using adhesive.

The screws will hold the boards to the post. Just make sure when the
boards warp they don't warp away from the post. Do that by looking at the
end grain and attaching the boards to the post with the end gain curved up
in the middle. When the board warps it will attempt to rise in the middle
but the screws will prevent that, so the edges will bear down on the post
which is what you want.

Avoiding glue gives you the option of replacing any boards that get
damaged or split with age.

If you want some sort of bedding compound between the boards and post, and
I wouldn't bother, then tar (asphalt roofing patch), tarred paper (roofing
felt), or liquid nails (polyurethane mastic in a tube) are good for that.
Liquid nails is an adhesive. All of those will keep water, bugs, or
whatever from getting in between the boards and the post. They are a lot
cheaper than epoxy adhesive thickend with microballons or wood flour.

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