Rather than tent suppliers, in Texas there a many awning suppliers. The decorative canvas awnings you see on commercial buildings and there are also those that take the same materials and make boat covers and furniture covers.
Another avenue to try are the people that make the large billboards that are stretched vinyl and printed on large machines. Or the sign companies retire the old vinyl signs and you could paint them yourself to suit.
All of these options are kind of expensive. I can't remember the square footage price and of course there are several different wieghts of fabric to chose from. Still I think going to a more industrial style paint would be cheaper and be les of a hassle than storing heavy tarps, rolling them just right and keeping them from molding in storage.
Since you already have a painted surface, your final coat of paint would be $50 a gallon for great enamels or traffic paint that comes in pretty much clown colors already. It would cover about 225 SF per gallon and cost 23 cents per square foot. The tarps and awnings are going or more like $4.50 to $6 per yard or 50 cents per square foot and they charge labor for tailoring on top of that, probably going after 85cents to $1.20 per square foot. Basically $39.00 per wall covered versus the good paint at $8 per wall covered, with you doing your own labor.
The over all feel of the canvas would be great in overall look. Still you are looking at $2400 per 1000 SF of haunt times 2 in the locations where you have to do both sides of the walls. End possible investment $5,000 versus $1000 per 1000 SF of haunt for paint.
Maybe just a few key scenes could be the canvas to get the look. It can be used as curtains to walk through or jump out from but not the whole haunt?
Still for research there are places in Jacksonville, Tx and Athens, Tx as there was once a big fiberglass boat building business in the area. Most larger sign shops will know who does awnings in your area because they usually get a logo and the tube steel structure gets custom fabricated and has to be set on the building with a small crane.
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