So I’ve finished the first of two arts and crafts style rockers using Paul Sellers plan. I finished it with shellac and a coat of johnson wax. The chairs go on my covered and screened porch. The chairs don’t get directly rained on but I’ve noticed some black staining creeping in on the back side of the chair that occasionally gets a little rain splatter if it comes down really hard. I plan to use poly on the second. Can I recoat the shellac and wax on the first chair with polyurethane?