Looks like a simple epoxy filler job. Use some tape...(wide masking can work but it will stick...could put some wax paper between it and the filler) to support a big blob of filled epoxy. I'd do it in two steps...first with high density filler or even milled glass fiber as the filler. This is placed in the more structural sections than need to be strong. Make it thick enough so it is like peanut butter...doesn't flow/pool on the tape. When set, file/sand back to the rough outline of the board's profile. Then re-mix another batch with only lightweight filler...use this to fill and undulations/air holes, and final profile. It will sand much nicer when set.

Paint if you're fussy ;-)