Gum mounting

Hello all. I want to mount my sheet of Gum Dichromate paper on some aluminium sheet for working on and washing and wondered what thickness to use ? I suspect 1 or 1.5 mm would enough but I don't want it to flex too much.

SPG

I'd suggest using perspex or acrylic sheet (2-3mm) rather than aluminium. That said, 1mm will be plenty rigid enough, although you'll probably find 1.6mm easier to find.