A flat gasket, like paper gaskets or flat rubber gasket, or sealing using a bead of RTV, yes, do exactly like 07JeepXK says; you want a clean dry surface on both sides.
Manufacturers are coming up with all different and more effective seals now a days, and you really need to follow the instructions for those seals and NOT general rules.
The only exceptions I can think of:
*"O"-ring seals, or something akin to O-rings, is becoming more and more popular, even between flat flange like surfaces. Again follow any manufacturer instructions to the letter, but absent any instructions, O-rings are seals you usually want to lube with a little engine oil or the fluid they are designed to seal in before installing them.
*Shaped Silicone Seals/Gaskets, again follow the instructions from the manufacturer, which usually states to NOT use RTV or any other additional sealant with it. I learned from experience, follow the recommendation, those silicone gaskets will leak unless you follow the manufacturer instuctions and then they seal better than most.
*Using RTV in combination with gaskets; this has been hit and miss for me, as many times it has improved the sealing, it has hurt the sealing and caused it to leak. I stay away from adding RTV to a gasket, unless its called for in the instructions, like in the tight corners of some gaskets where you run a single line of RTV. Either use the gasket as intended or a bead of RTV, NOT both.
The gasket installed as the manufacturer intended seems to work best, the most amount of times, everytime I've thought I was smarter than the manufacturer and started improvising my own improvements, I more often than NOT ended up creating a leak in the gasket.
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