Major marine catalogs think that real sailors don't need to make their own gaskets.
Auto supply has only engine related gasket sheet.
If you invent a company name for yourself (SV Bluenose, eg) I bet you can order from McMasterCarr. They have miles of foam and rubber sheet in all kinds of thickness in synthetic and natural rubber, much of it potential gasket material.
Much you can get in one foot squares, sometimes you have to get a square yard. Some of it is pricey, some very cheap.
High strength Weather Resist 40A (relatively compressible) EPDM rubber 12" X 12", $5.24. might be useful.
EPDM is an inexpensive outdoor rubber. It does not come closedcell foam by itself, but blended with other rubber.
Blends of neoprene,SBR, EPDM is the most common commercial closed cell gasket material with moderate resistance to fuels and weather. EPDM holds up better outside than neoprene (door gasket) but should not be in direct sunlight.
ECH, epichlorohydrin, is a closed cell rubber that will do moderately well with fuel and has excellent resistance to sunlight, heat, oxidation, weather, ozone. Gets my vote for all-round versatility.
Ionomer foam sheet reads good. Doesn't come thinner than 3/16" and they make buoys out of it. You want weather (Ozone, UV) resistant. Closed cell foam would give a firm more compressible gasket. But you have to get rubber that is rated for compression recovery. In other words, material that won't easily take a set and get tired while sealing a fitting. Don't want to compress any foam gasket more than 20% to 30%. MCMCarr has good charts and a bit disappointing explanations of uses.
And its a trip. You can start it on Page 3316!
HYLOMAR UNIVERSAL BLUE is a polyester-urethane gasket dressing that is a NON-SETTING compound you can use around the boat for other things you need to take apart later, and galvanic separation.
Idea is to fill the mini imperfections between the gasket and the fiberglass and/or metal surface. Better leak insurance than just depending on tightening up a dry gasket joint.
Believe it's distributed by Permatex and available in Permatex displays at the auto supply.
It's good for water and fuel and is NOT SILICONE based.
Don't have to scrape this dressing to get it off - easily removed with acetone.
As I see it, while synthetic gasket rubbers are getting longer lived, they still have to be replaced once in a while, especially electric fittings - so why not make it easy on ourselves.
Cut out thinner gaskets with X-acto knife and cardboard patterns. For round gaskets I just got a General gasket cutting tool that does inside and outside diameters at the same time. Some rubber like EPDM slithers around under pressure. You will notice in the online McMCarr catalog that some sheet is textured - this obviously is to create some tooth and to hold gasket dressing better for watertightness.
Foam stays put better for a seal. Dressing helps. You can get some dressing that acts like an adhesive and keeps the gasket from moving. May be necessary on freestanding fittings that capture the gasket only with fastening holes. Want the flange of the fitting to create a groove in the gasket, so some set is needed, that's why I lean toward closedcell foam material.
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