I use the 4" Eternabond. It gives better coverage.
You can apply it into the gutter, just make sure the surface is clean.
I have used Acetone and mineral spirits to clean the area before applying the E/bond.
Be careful if you use
Mineral Spirits.
I have never used the spray, just clean the area well before applying.
And, I just remove the loose Dicor. I will apply more Dicor to level the surface if needed, before installing the Eternabond.
Eternabond is a great product. It's not difficult to install but, take you time and clean the area well.
It will eliminate any leaks for a long time.