This next post comes with a disclaimer. I sometimes like to use permatex #3 aircraft gasket sealer. I'm not saying that this is best practice on a cooler reseal, but here's why I used it.
1. It has excellent oil resistance
2. It never fully cures, so there's no worry about chunks coming off into the oil or coolant (RTV is prone to this)
3. It offers some corrosion resistance as well. I've used this in the past on exterior bolts and it seems to prevent them from rusting. The idea here is to seal and smaller traces of corrosion on the O-ring areas.
5. It also worked as a lubricant for the seals while I pressed the assembly together.
The stuff is liquid (not a paste) and applies easily with the supplied brush. I only used a very thin layer.
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I used the same sealer to stick the header gasket on. I did NOT apply any sealer to the other side, since they already apply some kind of sealant from the factory and didn't want to mess with that.
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