Thinly slice the potatoes with a mandolin on the thinest slicing blade setting.
As you slice the potatoes place the slices in a large bowl of salted cold water and set aside.
In a medium saucepan melt the butter.
Add the flour, pepper, paprika and salt. Whisk until combined.
Cook on medium high for 2 minutes whisking the entire time.
Slowly add 1 cup of milk to the saucepan and whisk until combined.
Add the remaining milk to the pan and cook for 5 minutes, whisking until the sauce has thickened.
Remove the saucepan from the heat and add 1 cup of shredded sharp cheddar cheese and mix until combined. Set aside.
Drain the potato slices in a colander well, removing as much water as possible.
Grease the bottom of a 6-quart crock pot.
Cover the entire bottom of the crock pot with a double layer of potato slices.
Spread 1 cup of the cheese sauce over the entire surface of the potato layer.
Repeat 3 more times ending with a layer of cheese sauce.
Place a triple layer of paper towels over the top of the crock pot and place the lid on.
Cook for 4 hours on low. 30 minutes before the end of cooking time spread the remaining cheddar cheese over the top and replace the paper towels. Cover and cook until the cheese is melted and the potatoes are tender.