You might try painting the outside of the casting with polyurethane clearcoat with an isocyanate hardener added. (Similar to the old DuPont IMRON) Here's an example of the hardener I'm thinking of.

www.ebay.com/itm/374191568234?

Caution- - - -isocyanate hardener can be toxic- - - -cover all exposed skin and use a respirator while spraying it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!