Wish I had more intimate firsthand experience on this but there are a few others threads on this topic you may find helpful, one of which is included below. My hunch is this gap would be "taken care of" if you pulled apart the whole frame and re-sealed. If you caulk over the split it may work but doubtful it'd be a satisfactory long term fix - but could be wrong.
In regards to adhesives/sealants for ports I like butyl tape and sikaflex 295-uv products. Welding the split seems impractical as it'd permanently alter the frame and render it non-serviceable (welding the aluminum would likely require spool gun mig/tig capabilities) - this doesn't seem as though it'd do much to hinder water penetration to boot.
best of luck with your water proofing!
Still working on our "Never Ending Project" of refurbishing our E-32 Mk II ("CaryOn"). We're currently replacing all of the rubber seals on ALL of the ports. The smaller ones went fairly easy (not really) but the larger ports/windows are a REAL problem on the small hard radius corners. The...
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