If you absolutely insist on trying to reuse a manifold gasket, carefully apply Permatex #2 to both sides, thin coat and on every mating surface you can reach -- especially around the square holes where the intake actually happens. There is no compression to worry about -- that would be the head gasket -- the only thing going on is vacuum, and atmospheric pressure isn't too strong -- chances are good it will last a long time. But it's a much better idea to use a new gasket with Permatex.

You must be a troll.

"For some reason I have to take of the intake manifold off the head."

You don't know why you're taking the intake off, but you know you don't want to buy a 5 dollar gasket.

...perfectly normal, perfectly natural.

NO! NEVER,EVER reuse an old gasket unless it's the middle of the night out in the desert and you have no other choice. I don't care how good that gasket looks, once it's been used it needs to be tossed. Torque readings, with an old gasket, are totally unreliable. Don't take a chance on ruining something and/or stranding yourself in the middle of nowhere. Replace the gasket.

Do not re-use an existing intake manifold gasket on anything! The compression value is gone!

It's 91 civic with 1.5L engine and 4 speed tranny. For some reason I have to take of the intake manifold off the head. I did it carefully so that the original gasket was intact (at least visually)...Because I am going to make a 800-mile trip I want to make sure absolute reliability. is It a good idea to apply silicone liquid gasket on the mating surface and the existing gasket (after I clean them of course)?