1) If one has something on one’s hands which does not allow the water to touch the hand, then the Laws of Chatzitzah (obstruction which stops the water touching one’s hand and thereby nullifies the hand washing) apply. Anything which is a Chatzitzah for Mikveh is a Chatzitzah for washing one’s hands. Ideally one should check one’s hands before washing to see if there is anything on them. However, if one has already washed one’s hands and then saw something on one’s hands, one is allowed to be lenient and assume that it appeared after washing. (Shulchan Aruch and Mishne Berura 161)
2) Anything on one’s hands by which one is not disturbed does not constitute a Chatzitzah. If some people would be disturbed by such a thing and others would not, then it depends on the person. If he is disturbed, it is a Chatzitzah, and if he is not disturbed, then it is not a Chatzitzah. Therefore, a painter who does not mind when his hands are colored, does not need to remove the paint before washing his hands. (ibid)
