I always put a liner in my hats. I often use cotton cloth or nylon/ acetate(the silky stuff they use inside jackets/ coats). Most of my aviator hats get a quilted liner(nylon or acetate over fleece). Sometimes I put a full leather liner into a mask, depending what a customer wants. I have labels with my bus. name custom made that I sew into the liner, for a professional finished look! Its extra work, but worth it for quality look and feel on your hats.