Sealing surfaces...Coolant Expansion Tank Leaks?

[Eddystone]

This may be a minor point, but perhaps something to consider.

I recently have done some carb rebuilding and gasket replacement on a 1964 Corvair that I own. One thing I have noted is that the metal surfaces the gaskets need to seal to are not highly machined and polished. There's only so much I can do about absolute flatness of the mating surfaces without milling, but it occurs to me that the surfaces need to be really smooth and without gouges and scratches. To that end, I have spent a lot of time with little bits of 600 grit sandpaper to give the gaskets (often paper) half a chance. I also admit to adding a very, very thin schmear or RTV because these particular cars are chronically leaky.

ANYWAY, I have one car that leaks at the cap on the coolant recover tank sometimes when it is under pressure. I removed the cap and inspected the gasket. Since the cap was essentially brand new, it was fine. When I felt along the sealing surface at the throat of the expansion tank, I found it was noticeably rough and would have been difficult for even a rubber gasket to seal to, even in a brand new cap of the best kind.

I got out the sandpaper and smoothed the mouth of the expansion tank, and I am feeling much more confident about that seal.

[abscate]

Well the point of a gasket is to seal those irregularities , I think anything at 600 grit polish should seal well ?

I like permatex products for sealing stuff.

On p80s, the water pump, thermostat , water pipe flange