Making water reach all of the parts to be washed when doing wudu
Question

Sometimes something sticks my foot that I cannot remove except by using some substance, which I do not have with me. What should I do when it prevents water from reaching the skin?
Praise be to Allah.The one who does wudu must make sure that the water reaches all of the part that is being washed, and not omit any area of it, even if it is only a small area. If there is something on the foot that prevents the water from reaching the skin, it must be removed before washing it, and that part is not regarded as having being washed unless you remove it.
The evidence for that is the report narrated by Ahmad in his Musnad (3/424) and Abu Dawood (175) from Khaalid ibn Ma‘daan, 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 top of his foot there was a dry 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. Imam Ahmad said: This is a jayyid isnad. It was classed as saheeh by al-Albani in Saheeh Abi Dawood.
The great scholar Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: what if a man has grease on his hand – is his wudu valid?
He replied: Yes, his wudu is valid on condition that this grease is not a solid layer that prevents the water from reaching the skin. If it is a solid layer that prevents the water from reaching the skin, then he must remove it before doing wudu." (Majmoo‘ Fataawa Ibn ‘Uthaymeen 11/147).
The Muslim must be careful with regard to his worship and his purification, by covering his feet, whilst working, with something that will prevent things from getting onto the parts of the body that are washed in wudu that would prevent water from reaching the skin, or he should keep with him the substance that will remove those things.
And Allah knows best.