I usually delete spam from my inbox without reading it, but recently an email caught my eye, touting the new Pressurex Tactile Pressure Indicating Sensor Film. I looked up the website and it looks like a handy tool for determining whether two surfaces mate well. The film has tiny “microcapsules.” When you tighten or clamp two surfaces together with the film in between, it makes the microcapsules rupture and makes a permanent picture of the forces on the surface.
Here’s an article showing the applications in assessing disc brake contact with the rotor. It seems like it would be great for any automotive assembly where you want even pressure around a gasket. It’s also a good way to diagnose any adhesive bonding problems, especially when the members are clamped as the adhesive dries. The site says they’ll provide free samples, and free is a great price.