Should I top coat un-dyed leather?

[Jaan]

If I make, say a belt with a lining that's un-dyed, should I use a top coat on the un-dyed side too? 

[YinTx]

If it were up to me, and presuming veg tan, I'd oil it then seal it to help stop any body oil/moisture from creating unsightly stains.  It will at least slow the patina process.

YinTx

[Jaan]

Yes, it would be vegetable tanned leather.