The black stuff mechanics use for bolts is called Loctite. It’s a brand of adhesive that is used to keep bolts from coming loose.
Other related questions:
Q: What is the black coating on bolts?
A: The black coating on bolts is typically a zinc phosphating. This is a coating that is applied to the surface of the bolt to help protect it from corrosion.
Q: How do you blacken bolt heads?
A: There are a few ways to blacken bolt heads, but the most common method is to use a black oxide kit. This kit will usually come with a black oxide solution and a activator solution.
Q: What are nuts and bolts coated with?
A: Nuts and bolts are coated with a variety of materials for different purposes. For example, galvanized coatings protect against rust, while other coatings may be applied for increased friction or to prevent galling.
Q: How do you blacken stainless steel bolts?
A: There are several ways to blacken stainless steel bolts, including using a black oxide kit, using a vinegar and salt solution, or using a commercial blackening solution.