What might occur if there is oil or grease on an old surface before painting?

When oil or grease is present on an old surface before painting, it can create significant issues with how the paint adheres to that surface. Specifically, oil and grease act as contaminants that interfere with the bonding process between the paint and the surface. As a result, if the surface isn't cleaned properly, the paint may not adhere evenly, leading to a flat or dull appearance. This occurs because the contaminants can create a barrier that prevents the paint from forming a uniform layer. In some cases, this can even lead to the paint peeling or flaking off after application.

Proper preparation of the surface, which includes cleaning to remove all grease and oil, is crucial for achieving an optimal finish. A clean surface allows for better adherence of the paint, leading to a more durable and aesthetically pleasing result.