HOW TO.......best sealer option for painted rock

Hello, quick question.
Getting ready to paint a rock for my grandson. I haven't done this in decades.

What is the best sealer for intended outdoor display? Considering Never Rust spray enamel and a can spar urethane options here on hand but Im not sure which would be best for a good protected seal. Maybe theres a better option?

Thought I would ask before possibly using a less effective choice. 💐THANK YOU VERY MUCH,💐