Which paint primer are you using?
Thinning: TotalBoat Epoxy Primer Thinner 200
Thinner/Reducer: TotalBoat Epoxy Primer Thinner 20...
Hmm, I think I'm missing what you're asking. The strop just replaces a bail.
The strop takes the mainsheet load into the boom. It's pretty much a vertical force with a bit sideways when the boom is way outboard. Not much horizontal force going into the mast.
Almost certainly 6061-T6 or some closely related 6000 series for 95% of mast/boom extrusions.
You can't (easily) extrude 5000 series especially big sections like mast and booms. More common in small sections like flat bars and angles. That's why you see 5000 in rolled plates and some stiffeners...
Folks - when you weld 6000 series aluminum (typical boom/mast extrusions) the yield strength drops significantly. Yield drops from something like 34 ksi to 20 ksi in the welded area. So you think you're making the boom stronger but you're locally weakening it. This is why most good mast fittings...