gEDA-user: Clearing mask to mount shields

[Darrell Harmon <dlharmon@xxxxxxxxxxxx>]

What is a good way to expose a ground plane for mounting a shield?
I have a board which needs to be very well shielded, and am planning
on making a machined aluminum plate with compartments for each 
section of the circuit. The board will screw to this instead of
standoffs. What is a good way to clear the soldermask in strips so
that the aluminum can be in contact with the copper?

PCB won't let me convert a polygon to an element. It will not allow
traces wider than 250 mils, so creating the pattern with traces is 
not a good option. Is there any way to use one of the spare layers
as a mask keepout layer? The only way I see right now to make it 
work is to create several traces side by side in the larger areas.

-- 
Darrell Harmon
http://dlharmon.com