Spoilboard surfacing - strange problem/result

[Nealieboyee]

Hi All,
I just finished squaring and making parallel etc my machine (again) and went to surface my UHMW/HDPE spoilboard. Have a look at the photo. I only took off 0.5mm using an 8mm 3-flute carbide endmill. This plastic tends to come off as a layer rather than being cut, when you're taking off so little, however my problem is certain areas of the spoilboard are absolutely perfect. Can't feel any undulations. But other areas are weird. After taking off the hairy bits left over with a chisel, its better, but still not nearly as smooth as other areas.

Any idea why this would be happening?