Yes, crud accumulating during storage is very common. Replacing or machining the rotors is not required, your local garage should be able to sand the nasty spots away quite easily.

 

I usually fix brake judder/vibration by removing the disks and power sanding the surfaces. They get pad material on the disks at various spots around the perimeter, either from sitting, or when disks and pads are hot from frequent stops and then sitting at a traffic light with the brakes applied.

I've always replaced brake pads when 50% worn. (Never have to replace the calipers that way.) When bedding new pads, after stopping the required number of times, instructions warn not to leave the brakes applied after stopping, or at least don't come to a full stop. This prevents the dreaded transfer of pad material.

 

Wheels have been off, disks sanded with P400 wet and dry. Right hand disk is now ok but still vibration on the left. I'll try again tomorrow but it looks pretty clean.

 

400 is too fine. 80 grit is the standard.

 

Sanded the hell out of both disks until they shone like a mirror (well not quite) but still got front vibration. When the disks are cold there is next to no vibration, but when hot it's almost brutal, bordering on dangerous. Love the look of the drilled brembos through the 20" Detroits, but is there a better upgrade option using the red R callipers?

 

This might be a factor.