Hi,

I can linear_extrude this font fine but as soon as I put it against a
flat surface or substract it from one I am in trouble.

The font is "Pioneerbrush" and I have attached it

-Daniel

Most likely the font has a self-intersection somewhere in one or more of
its characters, quite possibly very small.

Isolate which particular character causes the problem, then blow it way
up and examine it carefully.

It might also work to linear_extrude it and view it with View/Thrown
Together.  (Do not render it with F6.)  Look for purple faces.

Unfortunately, finding a self-intersection doesn't solve the problem -
it just shows that the font is misdesigned.