If it is knotty pine, seal the wood first with shellac to prevent sap
from bleeding through. Shellac is compatable with most materials for
overcaoting, and is itself a good finish for pine tongue-in-groove
walls. I'd suggest spar varnish overcoat if you need durability for a
damp area (ie, wooden window sashes that tend to get damp from water
condesate on the glass panes in a spa enclosure or bathroom, etc ).
The UV-proof poly that others have mentioned here is well suited when
your stain has a reddish color, since red pigments tend to sun-bleach