Being hardwood you wouldnt need any sealant . "Sealant" is used for holes, gaps, joins etc. ie, silocone (ie product - no more gaps or polyurethane product - Sika FC11). The best coating is paint! Coatings to protect hardwood (& softwood) for decking, furniture from UV and rotting allow the timber to breathe. Some people for go that and end up painting becuase that is the strogest.
In your situation you could leave bare or paint. Just make sure its dry and nice to have it sanded. Ive got untreated hardwood timber with no coatings that is like iron after 40 years old.
Make sure you always predrill any timber (especially hardwood) and countersink.