has anyone filled their alum mast with expanding foam? Seems like a good way to increase buoyancy and decrease water (weight) build up in mast.

No it will hold water just bung it like Axis or seal with Sika flex.

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FoilColorado said..
has anyone filled their alum mast with expanding foam? Seems like a good way to increase buoyancy and decrease water (weight) build up in mast.

No good, the expanding (Builders) foam will suck up the water and then you have trapped saltwater in an Aluminium mast. If you use Sika (Silicone) make sure you get all holes and possibly the ends of the screw holes. Water seems to leak in everywhere. Silicone is used to glue aquariums so it is just a matter of sealing all ends.