Outdoor Raw Wood for Counter - Needs Finish -Other questions inside. (New york)

I saw a cool post on Pinterest that I started to do. So far so good. I put 250$ in so far and im impressed. I had some bricks behind my shed so I put them to good use.

Today I put together the wood on top of the cinder blocks.

This week I will begin sanding. My next step would be stain and finishing.

I will be using 80 grit to to 140 to 400 grit with a belt sander. Unsure of staining and sealing for outdoors. ANY tickets would be appreciated(even if better sand paper combinations.)

Some questions I have. I am gluing and clamping. Some screws but I keep looking the bit in the wood so glue is working well.

Should I use wood putty with any of the gaps caused by warping or just leave it.

I need it to be able to stay outdoor forever. I may build something overhead but it will not be protecting against elements.

How do I apply the finish. Raw wood