Definition
A volatile solvent added to paint, dope, varnish, or other finishing materials to reduce their viscosity so they flow and atomize properly during application. The thinner evaporates after the finish is applied, leaving the solid film behind. Each type of finish requires a specific, compatible thinner; using the wrong one can cause the finish to lift, blush, or fail to cure correctly.
Plain English
A liquid mixed into paint or dope to make it runnier so it sprays or brushes on smoothly, then evaporates as the finish dries.
Context Anchor
Seen in aircraft painting, fabric-covering, and refinishing instructions, especially where a coating must be mixed before application.
Derivation
From the everyday sense of 'to make thinner' — the liquid literally thins out a thicker substance so it can be worked with.
Why Pilots Care
Correct thinning produces a smooth, durable finish that protects the airframe from corrosion and preserves aerodynamic performance.
Intuition Check
Do not read “thinner” as simply “weaker.” In aircraft maintenance, a thinner is a specific liquid used in a controlled amount to make the correct coating flow properly.
Example Sentence 1
The mechanic added the manufacturer-specified thinner to the dope before spraying the fabric wing.
Example Sentence 2
Using the wrong thinner caused the topcoat to run and fail to adhere to the wing surface.