tongue and groove ceiling
I would hit the tongue and groove lightly to see if the dust is coming from the wood. If it is, than the sealer would help. Do not caulk the joints or you will spend you life caulking as the wood expands and contracts. If the roof is above the dust may be coming from the sheathing material that the tongue and groove is amounted to. If you can't solve the problem with a sealer, I would use spar varnish if it is stained, than you may have to lightly caulk and paint it. Try hitting the wood first to be sure it is the ceiling first. It may not be coming from there.