How to remove Dicor for new AC install??

I started on my project today of installing a second AC unit in my trailer.

I removed the vent from inside, and then started on the outside section. I removed all the Dicor that was present on the flange, and then removed the 28 screws holding the vent in place.

Very slowly, I used a plastic putty knife and worked it under the flange and outside edges of the Dicor sealant. In about an hour I had the entire assembly off, with no rips or tears.

Now I need to figure out how to smooth out the roof where the leftover Dicor is. My plan was to smooth the perimeter area out, cover the perimeter with 4" Eternabond tape, and then set the new AC unit over the opening.

The remaining Dicor is really a mess. I used the putty knife and worked off as much as I could, but it is nowhere as smooth as I think it should be, in order to lay the Eternabond tape down.

Is there a secret to removing the leftover Dicor? I even thought about cutting the roof material away at the edge of the Dicor, then sealing the roof back up with the Eternabond. Will the Eternabond stick to the plywood roof? I really hate the idea of cutting the roof material, but I really need a smooth surface to sit the AC unit on.

Any ideas or suggestions?

Thanks,

Jerry