Do factory pistons have a coating on top?

[Eric Stage I]

I have the heads off my factory original engine, cleaning the carbon off the tops of the pistons. When I get the carbon off, the surface seems to be a greyish greenish coating. Is this just an effect of combustion, or is there a coating? It looks like I can scratch through it to bare metal. Has anyone seen this? I'd like to be able to polish up the entire combustion area for better detonation resistance (hopefully).

Thanks,
Eric

 

[REDS HOT AIR]

my 87 pistons had some kind of coating on them

didnt notice it on the 85 pistons

 

[PaulRV6]

When I bought new GM .030 over pistons two yrs ago they did not have any type of coating on them, either top or skirts.

paul

 

[Grumpy]

I have a 9 mile motor here....doesnt look like any coatings to me

 

[Eric Stage I]

Hmm, maybe just discoloration from 50k miles of combustion. I'll go ahead and polish 'em.

Thanks guys,
Eric