so I ended up finding an oem replacement copper one online for $1350 shipped to me. But unfortunately now I have a new problem. After pulling the tanks off of the old core I see that they have some pretty bad pitting going on, nothing is leaking under pressure but I was wondering if there is a way to coat the inside to prevent it from getting any worse ?

thanks for all the replies so far !

Cat sells coolant additive/rust preventative [red beet juice].

devcon epoxy. loctite norback epoxy is also a good choice. contact there tech advice line for advice on selecting the exact product. they have several formulas. i know they both have products specifically for repairing cast housings like pumps etc.

when the mechanics went through our d4 they coated the top and bottom rad tanks with devcon hasnt leaked hasnt changed and that was 10yr ago. i dont remember what grade they used but i know it was devcon.

we use a product called eco cool AOC1000 made by australian organic coolants. its available in concentrate or premix has 5yr service life. for all petrol, diesel and gas engines that dont require anti freeze protection. sold in 1L, 5L or 20L drums its quite economical too.

if you need freeze protection use AOC5000.

def need freeze protection haha, can get down to -55 C here.  Thank you for the suggestions ill look into it for sure !

Sent you a pm.  I cant seem to get any useful info from suppliers on this topic.....do you know where to contact devcon directly ?

I asked this question a few years ago. Ccjersey recommended not painting it but also said that Glyptal is suitable.
www.acmoc.org/bb/discussion-d72/25473-ra...or-tank-paint#207611

itwperformancepolymers.com/technical-service/ I use the titanium one for block repairs. It is rated for 350 degrees. I have done two and had great results.