I had the same separation on the left side of my d28 some months ago and I fixed it then with CA glue ... but it separated again as I can see today.
I´m storing my D28 most of the time in its bluish case. The guitar seems to be from around 1970.
I´m going to fix the binding with CA glue again and hope it will be ok for the coming decades...
Or should I use another glue?
