I understand your concern, but I never had a problem. I used a bolt on neck but a good dovetail should register just as precisely (ie hold firmly without glue) so you could unmask both mortice & tenon 3/4 of the way through finishing, dry fit & do some leveling of the seam at that point, then disassemble/remask/finish coat etc through buff out before final ass'y. (You'll have to floss the joint of overspray a bit after finishing, but no big deal.) I always thought the trade off of feeling that seam a bit was well worth the benefits of doing them separately. (In theory, with a glued joint, you could drop fill the seam after glueing & eliminate the seam) (Make SURE the masked area on the body is smaller than the width of the heel ! ..... go ahead, ask me how I know this
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