...and Interlux tech support a mechanical bond was the only way I could go. I believe I had to do with the solvent packages involved in the Hydrocoat ECO.
They did say that when you get it right a chemical bond works very well but they said that there are many variables and that when I goes wrong.....
The other reason I sanded between the barrier and the bottom paint was that by the time I had all that barrier on, it had the texture of a basketball!
I am trying for a 400 grit finish in the end. I wanted to thin the barrier and apply with a foam roller but tech support said NO!
I used 5 coats mostly because in the end I sanded more than one coat off. I know because I alternated colors of InterProtect.
Bruce