What conditions are necessary for ghusl to be valid?

It is necessary for women to wait for the menstrual and puerperal bleeding and for men to wait for the sperm to stop completely.

It is necessary for a man to sleep for a while or to walk a few steps or urinate even if a little amount in order to make the sperm flow stop fully. If a man makes ghusl without doing any of them and sperm comes out after making ghusl, his ghusl is not regarded valid. He has to make ghusl again.

The second condition for ghusl to be valid is to wash the whole body. The recommendation mentioned in some books that not even a place as little as pinhead can be left dry should not be understood literally but as washing the whole body thoroughly with water.   

Dental filling and plating, and materials like paint and similar things on the skin that prevent water from touching the skin and that are hard to remove do not prevent ghusl. Therefore, if the material dirt on the body is cleaned by using soap, the remaining materials like paint, dough, etc that prevent water from touching the skin and that are hard to remove do not invalidate ghusl. The dental filling and plating are regarded like that, too.