Heavy coat of urathane (paint/spray 3-5 layers) should do it. You could also use vinyl shelf/desk liner (the kind that sticks) and silicon sealer to seal the seams and cracks. Keep in mind that any corner or ledge the rats can get their teeth on is likely to be chewed as well.

Its a beautiful cage but most likely will need to be replaced in a few years just from chewing alone, depending on how much ledges are in reach. Smooth or curved edges/corners are not as easy chewed. But something to watch out for as time goes by.
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PHLdyPayne