You might try glazing putty for thinner skimming instead of bondo, and use a filler primer to cut down on sanding. It's obvious you know what you're doing but I used to work with aircraft parts (making them out of composites and repairing/painting them) and these were some of my tricks.
How long overall did it take you to rebuild/refurb that head?
I do wanna "steal" that trailer tho
Good job.