The Willamette material is pretty consistent. You may want to check your core and skin consistency. We have found Trupan, Ranger board, Willamette and Plum Creek to be consistent. Working with Masonite MDF can be more difficult. A grit of 150 to 220 seems to work well, with a finer grit, around 400, to finish for profile wrapping.
Carbide is especially prone to heat buildup during grinding.
We normally recommend grinding carbide back clearance at 12-15 degrees. More than that won't allow enough backing material to provide adequate knife support. Both WKW's and Leitz's systems have pros and cons. The thicker carbide seems to reduce breakage compared to other carbide knife systems. We have customers successfully using both systems.