Re: Glue for birdsmouth mast?
The reason for using epoxy with a birdsmouth spar (and the reason birdsmouth spar construction wasn't widely used, even though it's been around for nigh on a century at least) is that most adhesives -- e.g., casein, urea-formaldehyde, resorcinol, etc. -- require (1) well-fit joints, (2) and the correspondingly thin glue lines and (3) relatively high clamping pressures.
Epoxy, OTOH, is happier with relative thick glue lines and relatively low clamping pressures. It's also quite good at filling gaps and so isn't as dependent on well-fit joints. Epoxy also can be tweaked with additives -- microfibers, fumed silica, wood dust, etc. -- to get the desired consistency and strength needed for the job at hand. Further, unlike other adhesives (which see above list) epoxy has a relatively long open time, so you have some time to get your spar assembly together, adjusted and aligned before it needs to be clamped down.
The one argument for using an adhesive other than epoxy is that epoxy degrades under UV light. Presumably you will be varnishing or painting your spar, so that is in reality a non-issue.
Birdsmouth joints are not hugely precise. Consequently, it wasn't used prior to the advent of adhesives like epoxy, despite its advantages for assembly (self-registering), since the resulting spar would not be as strong as one built with traditional staved construction.
There is no reason to build a birdsmouth spar with an adhesive other than epoxy.
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