Pretty simple. On the gasket surfaces, it is not (or should not with a good gasket) be in contact with fuel. I works on threaded fittings on fuel lines, because again, it is not directly in contact with fuel.
If it is in direct contact with fuel (not gasoline) it partially dissolves into a gummy mess.The same is true for many other sealants with todays fuel.
On Weber gaskets to insulator / isolators where I use it, I clean the port itself with lacquer thinner before the Permetex set up on the inside so it will not break down and be sucked into the engine. If the gasket is good and surfaces flat, minimal sealant is required and essentially none is in direct contact with fuel.
If this stuff was used to seal a seam inside a fuel tank it WILL fail.
I hope that helps. .
I guess the best answer would be to use it sparingly to fill voids in gasketed areas away from direct contact with fuel or solvents. Hows that? I hope this clears up how I use it.
Best, Gordon