I'd be reluctant to use ANYTHING before it's glued....sealer seals glue, too.
However, major subassemblies that are likely to get dunked (basically anything in front of the spar) should be sealed. I use water-based polyurethane varnish as a brush-on, or the stinky spray-can poly if I am spraying it. Polyurethane is very chemical-resistant, and covering sticks well to it. I also use WBPUV as the adhesive when I glass the floats or hull of a plane. Not toxic like epoxy, and plenty strong. It will Wash-off if I screw up, instead of soaking stuff in solvents.