If this J channel is an extruded aluminum (the thicker, usually not painted) drip rail, I would use butyl putty tape. This is like the old style window putty tape, only the sealant is a hundred times better. You can get it online , locally at RV parts dealer and some specialty shops.
If this is a rolled aluminum (thin sheet bent to shape, usually painted) mini gutter, I would replace it with the extruded drip rail and tuck a regular aluminum gutter under it.
Today is PERFECT!
All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.