Michael & Brad,You may like HDU so much that you never go back to redwood.
Coastal Enterprises publishes an informative booklet on fabricating and finishing. Awhile back somebody said it was even online, too.
I also agree with those, like Dave, who prefer not backing up their HDU with MDO. It's just too many letters.
I've installed single face foam signs in several different ways to compensate for the flexiness of the material.
One way I like uses square tube for posts, 3" or 4". Weld horizontal stringers of inch or inch-and-a-half square tube. Four horizontal stringers can yield approximate centers of 16" on a 4'x8' sign. I've even done this in a knock-down version by welding stubs of inch-and-a-half tube to the posts, for accepting the one inch stringers. This way I could transport and install it by myself by carrying it in pieces and assembling it on the lawn on site, pulling it together with pipe clamps, then raising it by hand, leaving the clamps holding it together till the concrete sets up in the holes.
Another configuration I have used is an entire back-frame, not just stringers, of one inch square tube. The perimeter of the frame could match the blank, with center braces on 24" centers or less.
This back-frame I have also made out of one-inch square aluminum tube and no welding. At the corners I use a plastic elbow from Outwater Plastics. It perfectly fits the aluminum tube I get locally from the steel yard. For attaching the vertical braces, Outwater has plastic 'T' fittings. I secure all the joints with stainless screws or pop rivets.
When I make the frame of aluminum, I attach it to the back of the HDU prior to installation on the posts. If the frame is steel, it's easier to weld it all together, then paint and install, hanging the sign blank last.
For fastening, quarter inch stainless through-bolts with a carriage head looks nice. If you don't want to cover the whole face with bolt heads, just use them at the perimeter, or even just the corners. Then use large thread sheet metal screws, such as a #12 or #14, into the back of the foam from behind for additional attachments. Run these screws in by hand, not with your 18-volt cordless. Also, I believe that putting some epoxy on the threads before you put them into the foam will add strength.
If you want totally blind fasteners, you could glue the foam to the frame, then countersink and fill bolt holes at the corners and/or perimeter, using the knife consistency filling compound.
One advantage of using a skeletal frame of tubing is in case you want the back to be visible. If you are marblizing, for example, it can be very striking for the entire back to be marblized. You can also lightly etch into the back, very inconspicuously in a corner: "Florence Marble Works", in Italian, or some such nonsense, for a final touch of faux.
Brad
Paris, Arkansas,
home of "Paris Marble Works". 
By using square steel posts
You can take advatage of all the decorative finials, caps, ornaments, etc., made in cast iron, cast aluminum, or brass. These are made by many different companies as fence components, and fit different sizes of square pipe. Rosettes, coner brackets, etc., can be welded, drilled & bolted, pop-riveted, or, in the case of a cast iron finial, simply siliconed to the top of the pipe. These ornaments can often make a plain installation into something very beautiful, even ornate-looking.
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Brad Ferguson
4782 West Highway 22
Paris AR 72855
501-963-2642
signbrad@cswnet.com
[This message has been edited by Brad Ferguson (edited February 25, 2000).]
With this 8'x10' monster we quoted I can see how the alumilite backing would be a much better choice. Im not a big fan of MDO anyways. Heard to many horror stories with the MDO. As far as the steel structure, you had some good ideas. I am a former welder/fabricator and am sure it will be overbuilt a bit, but with an 8'x10' sandblasted sign inside we dont want it going anywhere. Thanks for the advice.
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