Thermostat Housing

I replaced the therm. on my 94 Sub. and it has the rubber o-ring. I didn't use any sealant on the housing when I assembled it. It is now leaking a week later. Should I have used a paper gasket with it or some sealant when I replaced it? There didn't appear to have any sealant or gasket on it when I removed it.

Should have replaced the o-ring. Also make sure the manifold surface is perfectly clean.

I installed a new o-ring with the therm. I just wanted to be sure I didn't need to install a paper gasket or anything.