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I have tried a lot of different gaskets, including paper gasket sheet, liquids, teflon sheet, and nothing at all. I usually go with the 1/32" teflon sheet as first choice these days. Many have advised oiled $1 bills as they are made from cotton. For slide valves I've generally been happy with milled surfaces on the ports and 1000 grit paper on the valve.
I use an oiled brown stock “grocery bag” . Now when I say oiled, I don’t mean that I soak it in oil, just; rub it with my really oily fingers. I’ve seen Dad and grandpa stack several together to use for thermostat housings and such. Really, if the mating surfaces are true, don’t take much.Cletus