Many modern weapons like missiles and bombs have a sensor on the nose for Infra Red (IR), Laser, or GPS guidance. Typically the sensor is mounted behind a coated lens to protect the sensor and allow it to have a wide field of view. Many times modelers paint them using gloss paint. The below tutorial will show you how to replicate the coated lens appearance.
To start with, the tools required are a drill bit the size of the sensor, Liquitex Acrylic Gel medium (or similar product), silver or chrome paint (I used Model Master Chrome) and Tamiya X-19 Smoke paint. The smoke paint is a transparent black paint.
Here is a photo of a Russian R-3S missile in 1/48 scale from a MIG-21 kit.
Take the drill bit and drill into the missile to create a concave hole.
Next take the silver/chrome paint and paint the inside of the area. This is done so when complete the light will reflect thru the transparent black and give it depth and look like an actual lens.
After the paint dries, using the acrylic gel form the shape of the nose. This may take a couple of layers to build up to the correct shape. The gel is a milky white but dries to a golden clear. Some sensors will have a golden coated lens so you can stop here. If the lens has the black tinted coating, proceed to the next step.
Once the acrylic gel dries, paint the outside with the Tamiya Smoke. The level of tint can be set by the number of coats you paint on. This is after 3 coats.
While it is difficult to show in a photo, the light will reflect and give the sensor lens a very realistic effect. This method can also be used when making FLIR camera lens’ as well as targeting turrets on targeting pods of airplanes, helicopters, and vehicles.
Here is a quick video close to light source.
Kit missile on left, Drilled, chrome paint and acrylic filled, and final on right.