I would taper the relief rather than have the relief end in an abrupt
cut edge 3 inches in from the end.
Is the bulkhead going to be painted? If it is then is won't really
matter if there is some variation in the inboard edges of the fillet.
I hate routers. noisey, dusty, too fast, and hard to control unless you
set up a jig.
I'd draw a line three inches back from the edge and hand grind a taper
with a disk grinder( also noisey and dusty but easier to control by
hand) being careful not to take off any wood beyond the
line. Use the glue line in the plywood to judge eveness of depth.
You could also bevel the edge off with a power plane or even a hand jack
plane if you are good at sharpening one.(sharpening the blade is the
secret) If you have a lot to do the hand plane would be tedious.
All this assumes that you are bonding the entire perimeter of the
bulkhead to the hull rather than just short little tab angles. I've seen
it both ways and I can't really tell from what you wrote. If using
multiple short tabs, I'd use a really sharp slick (a wide flat chisel
with a long handle) to bevel slots that the tabs would be built in.

By the time you figure out how to build a jig for the router, hand
grinding it with a disk sander would have it done.
The inboard edge of the glass "fillet" can be faired with micro baloons
and epoxy but if you use strips of glass cut from a sheet of fabric on
the diagonal, you will not have to deal with the hard edge of glass
tape. Cutting on the diagonal keeps the fabric from fraying and orients
the fiber correctly for this aplication.
Actually, you can fillet the bulkhead to the hull without cutting a
recess at all. The glass is not all that thick that it can't be faired
to the flat of the plywood so that it can't be seen if painted. Either
way will require some fairing and sanding to achieve a smooth joint.
SteveJ

mike worrall wrote:
Imagine a freshly cut-out plywood bulhead lying horizontaly on a table
before you. The bulkhead will be attached to, or installed into the
fiberglass hull using 'bonding angles' i.e., several layers of 'glass
/ epoxy that will lap onto both the bulkhead and hull. I'd like to
relieve, or remove, about 1/8" of the plywood (face) material all
along the edge of the bulkhead (where it will bond to the hull) for
(say) 3" from the edge on both sides of the bulkhead. In this way,
the 'glass angles will lie flush with the bulhead surface.

How do I do this? That is, wat tool(s) would be used for removing the
plywood, leaving a clean cut of consistent depth?

Appreciate any tips.

Mike Worrall
Los Angeles