Blistering to inside of left hand valve cover

You really are making mountain out of nothing.

Metal does not form flexible bubbles. Paint does. Clean away the failed paint and be done with it.

Your issue is on the inside. The engine is made of bare metal on the inside. Oil wont hurt it.

I'm afraid you've lost me. In particular the metal-bubble comment.

In any case, I replaced the valve cover which was a 'mountainous' 15 minute job. Good opportunity to helicoil the vavle cover guard mount holes that have corroded and been leant on over the years. A horrific total of 45 mins including a cup of tea.

In terms of the substance of the disucssion above, all I can do is reiterate that the interior surface of the valve cover has had a finish applied as per the photos. It is not bare metal. My decision was not to puncture this finish by removing the blisters. You may well choose to do otherwise, but I am struggling to understand where you think my rationale is wrong.

Personally I think the two viable options were: leave as is on the basis that the blisters are unlikely to fracture; or replace the cover. Worst option is to remove the blisters thereby puncturing the factory-applied finish. In case of interest, and I think it points to the same conclusions:

https://ktauniversity.com/blistering-bubbling-coatings/