Substances that Prevent Water from Reaching the Skin in Wudu
Question

I want to know how I can distinguish between makeup that prevents water from reaching the skin when doing Wudu and that which does not have that effect. I know that anything which forms a layer, such as nail polish for example, prevents water from reaching the skin, but there are cosmetic preparations that do not form a layer; rather they are oily to a large extent, such as lipstick and foundation cream. Do these prevent water from reaching the skin? Does everything that is waterproof – and these types cannot be removed by water only except with difficulty – prevent water reaching the skin when doing Wudu, as I have read in some chat rooms? Do powders not prevent water from reaching the skin, such as powders used on the cheeks and eyeshadow? Is there a practical way to distinguish between the two types?
Praise be to Allah.Water must reach the skin in Wudu
It is essential that the water reaches all the parts of the body that are to be washed when doing Wudu . The evidence for that from the Sunnah includes the following:
It was narrated from Khalid ibn Ma`dan (may Allah have mercy on him), from one of the Companions of the Prophet (blessings and peace of Allah be upon him) that the Prophet (blessings and peace of Allah be upon him) saw a man praying, and on the back of his foot there was a spot the size of a Dirham that the water had not reached, so the Prophet (blessings and peace of Allah be upon him) instructed him to repeat his Wudu and his prayer. (Narrated by Abu Dawud (175); classed as authentic by Al-Albani in Irwa’ Al-Ghalil (1/127)
The four Schools of Fiqh agreed that in order for Wudu to be valid , it is essential to remove anything that may prevent the water from reaching the parts of the body that must be washed. (See: Al-Mawsu`ah Al-Fiqhiyyah, 43/329)
What are the substances that prevent water from reaching the skin? 
The definition of the substances that prevent water from reaching the skin is based on real-life observation. The scholars pondered this matter, and concluded that substances are of two types:

Substances that have “mass”, i.e., they remain in the form of a layer, so when they are placed on the body they remain as they are, forming an integral mass, and can be removed by peeling or wiping.

Examples of that include waxy substances and paint. With regard to substances that have mass:

Either they are not waterproof, such as some types of makeup and powder . In this case, when doing Wudu, it is sufficient to rub the skin well, until it is thought most likely that the water has reached the skin.

Shaykh Al-Islam Ibn Taymiyyah (may Allah have mercy on him) said:
“With regard to rubbing the body when doing Ghusl or rubbing the parts of the body that must be washed when doing Wudu, that is obligatory if it is not known for certain whether the water will reach where it should without rubbing it, as in the case of inner layers of thick hair. But if water may reach where it should without rubbing it, rubbing it is encouraged […] By means of rubbing, proper cleansing is achieved, and one may be certain of having made the water reach everywhere it should, which is obligatory. Hence it is prescribed, as in the case of rubbing between the fingers when doing Wudu." (Sharh `Umdat Al-Fiqh  1/367-368)
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
“What is required when doing Wudu and Ghusl is to make sure water reaches every part of the body that must be purified. As for rubbing it, that is not obligatory, but it may be essential when there is a need for it, such as if the water is very cold, or if there are traces of oil, paint and the like on that part of the body. In that case, it is essential to rub it, so as to be certain that the water has reached all the parts of the body that are to be purified." (Fatawa Nur `Ala Ad-Darb  3/94)

Or that substance may be waterproof, as in the case of some kinds of makeup. In that case it must be removed before doing Wudu, so as to ensure that the water will reach the skin when doing Wudu.

An-Nawawi (may Allah have mercy on him) said:
“Our companions said: If he puts fat or wax or dough on the cracks of his feet, or puts henna on them, and the “mass” or substance of that remains there, he must remove it, because it prevents the water from reaching the skin." (Al-Majmu`  1/426)
2. Substances that have no “mass”, i.e., they do not form a layer; rather when they are placed on the body, they are absorbed by the skin. Nothing is left of them; rather they may leave a trace such as colour, for example.
Examples include most creams and oils, traces of kohl and henna, and so on. The presence of the substance does not invalidate Wudu, but if the substance is fatty or oily, then the one who is purifying himself must rub the skin well, until he thinks it most likely that the water has reached it.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
“But if the oil does not have any “mass”, and only leaves a trace on the parts of the body that are to be washed, then that does not matter. But in this case, it is essential for the individual to pass his hand over that area, because usually the oil repels water, so perhaps it may not reach all of the part that he is washing." (Majmu` Fatawa Ibn `Uthaymin  11/147)
And Allah knows best.