I haven't heard of "milk paint" but Google certainly has!

1870 Milk Paint Formula

* 1 Quart skim milk (room temperature)
* 1 Once of hydrated lime by weight (Available at building centers. Do not use quick lime, as it will react with the water and heat up. Hydrated lime has been soaked in water then dried.)
* 1 to 2 1/2 pounds of chalk may also be added as a filler.


http://www.milkpaint.co.za/default.asp

http://www.pioneerthinking.com/milkpaint.html

http://www.realmilkpaint.com/products.html


Can't at this point find any particular reason to recommend its use on RC models but who knows...