tanned leather, smooth leather
靼 centers on processed animal hide: tanned leather, especially when smooth and finished.
靼 combines 革 (leather) with 旦, which likely contributes the sound. The exact historical role of 旦 is uncertain.
Leather 革 is tanned and smoothed until it is as bright as the morning sun 旦: 靼 is smooth tanned leather.
For タン, imagine a tan-colored piece of smooth leather: tan -> タン, and the leather is perfectly tanned.