I just bought one last week and was painted inside - I did find a helpful guy on YouTube that will take them and sandblast them along with rebuilding - he also shows you how to do it along with parts list
It's an anodized coating and this is what Merc said when I asked the question
The inside of the gen 3 cool fuel modules were always coated and still are today. There is a lot of misinformation on the internet. In about 2015 we came out with a new top cap which is anodized and is resistant to today's fuel additives.
