Alumalite is quickly becoming the standard for exterior signage.  2 sheets of Aluminum laminated to a corrugated plastic material make this exiting new product rigid, instead of flimsy like a non-laminated sheet of aluminum.  This gives the face of the sign a flat, smooth look, along with the baked-on Gloss Enamel finish, topped off with the vinyl lettering of your choice, and a plastic edge cap around the perimeter,  plus plastic h-channel to hide unsightly seams between sheets on larger signs.  Wood signs, or MDO plywood, are still very popular amidst the new alumalite material.  They work well with custom shapes, dimensional signage, double or single sided as well.
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