Short Answer

If anything prevents water from reaching a place that needs to be washed then wudu or ghusl will be invalid. For example, glue, paint, or nail polish are all examples of something which prevents water from reaching underneath. Things like oil, lotion, hair dye, and petroleum jelly does form a barrier but does not usually prevent water from reaching the skin or hair. Some makeup consists of a perceptible barrier which prevents water and other types of makeup does not.

If a ring or watch is very tight it will prevent water from getting underneath to the skin. These things may be slightly moved while washing to ensure water reaches underneath.

If there is a need for that thing to be there, such as someone having braces on their teeth or a permanent wig on their hair, then water flowing over that thing suffices.

If you prayed without wudu because you didn’t notice something like glue or paint was blocking the water, you must repeat the prayers you did with that wudu.

Version 0.25

Shaykh Mustafa Umar