I wonder if using some sort of sealer, like those thin pre-stain
conditioners would help in painting yellow pine.

I really want to spray these pieces I have and the pine really soaks
up the paint.

The real problem though is the end grain which really shows through.

I don't want to spackle because I may have a large qty to do.

Any techniques for "production painting" yellow pine and getting a
nice HARD coat?