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How critical is the condition of the angled surface that faces the pump vanes. (12 vane)
My spare housing is quite badly pitted in one area,-re-usable or is it scrap?

I have no real idea how much pitting would reduce water flow, but my guess would be very little. However it should be easy to fill the pits.

I once had a 6HP Petter marine diesel raw water cooled generator in a small island ferry. The actual sealing surface of the alloy cylinder head was corroded, probably due to a mixture of copper gasket, alloy head & salt water.

I removed any corrosion, & thoroughly cleaned the surface exposed, & filled the resultant recess with marine grade epoxy filler. I did this in the hope it would run long enough to get a replacement head from the UK to Australia.

6 years later the replacement head was still sitting on the shelf, & my patch up was still working.

Done properly I am sure a good quality epoxy filler/putty would hold up in the much cooler area of a water pump.

Hasbeen

Howard722 wrote:How critical is the condition of the angled surface that faces the pump vanes. (12 vane)
My spare housing is quite badly pitted in one area,-re-usable or is it scrap?

Guessing you mean the water pump cover, there are lots about and they come up for sale pretty cheap all the time.
Be aware that the covers for the 6 vane and 12 vane pumps have a different angled surface and are not interchangeable.

I had my water pump reconditioned by Tony Hart and he said the pitting on my pump cover, which is quite bad, doesn't matter....

Pitting of the housing (or impeller) is usually not caused by corrosion but by cavitation (imploding air bubbles).
Once you have pitting it will only be amplified to a point where the pump doesn’t work properly anymore.