Keep in mind Yakisugi ("shou sugi ban" is a Western misinterpretation circa ~1995) is traditionally performed on wood which is already weather resistant, particularly Japanese Cedar ("sugi" is cryptomeria japonica, a type of cypress endemic to Japan). Even then they still treat it with oil regularly to improve its longevity. So I wouldn't char some white pine and stick it outside and expect it to last.

We still treat wood with oil. Your local home improvement store should stock a bunch and you’ll find more at a woodworking store.

You can also add a sealant on top like polyutherane if you want it to last even longer at the cost of imparting a different texture.