how to stiffen a hat brim.?

You can sew a piece of narrow matching bias tape on the under edge of the brim to create a casing, and push a piece of slim boning or a thin gauge wire into it long enough to go all the way around the brim edge.

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how to stiffen a hat brim.?

I have a wool felt hat with a large brim and it is too floppy. I have a felt hat stiffener spray but it is not doing the job, is there anything else I can use to make the brim more stiff.

Well, you could try a better stiffener. I've not had need of it, so I'm not sure how well it works, but millinery supplier Hats by Leko carries several sizings. http://www.hatsupply.com/sizingstiffeners.htm

I also like the idea of sewing in a wire around the edge, but it will visibly alter the hat.

You could also try reblocking the hat.

Alternatively, if you don't mind almost completely losing the hand of the fabric, you could use rabbit skin glue. That stuff will stiffen ANYTHING. Of course, there are the bunnies to think about, too....

How To Stiffen Felt