Yep, but there must be another trick? I want to do 20 doors, I will be block sanding forever.You are right. White shows every scratch, black is even worse. Try priming with a high solids primer and then block sand by hand.
Wurth wood group, A&M supply will carry the double refined MDF. I use it for my panels sanded to 400 grit, scuff sand in between coats with 320. After widebelt sanding, RO sand to 180 to get out the cross grain scratches. 2 coats of hi solid primer followed by 1 top coat of color finish.
I've been told wurth will sell to walkins, in Norfolk at least they do.