Leaking and using a liquid type gasket

I recently changed the thermostat on a 1994, 1.3 toyota corolla.

The thermostat has a rubber ring that sits around the thermostat and the two halves of the housing join up meeting at the rubber ring. In the thermostat box was no gasket of the size I needed and the Haynes manual makes no mention of using any gasket other than the rubber ring.

On one half on the inside was a small flaw in the casting, just a small lump which sits under the rubber ring which I still dont think is really significant. But there is a very slow leak, below the thermostat housing.

Should I have applied some sealent of some kind to the two flat metal faces? If so what kind of thing should I get? Thanks for any advice.

go to a dealer and buy a new seal

Ordinary domestic silicone from your local DIY shop will do fine. You need to let it cure for a few hours before filling back up with water though. Probably cheaper to just get a new O ring though.

Thanks to all. I will get a new O ring. Would that silicon sealant be applied straight to the bare metal surfaces?

I had a little seep after changing a thermostat on my astravan, even with the new rubber seal. I smeared some Hylomar Universal Blue sealant around the metal surfaces which did the trick.