How a tiny bit of lacquer grounded new Falcon 9 rockets for a month | Ars Technica
https://arstechnica.com/...nasa-and-spacex-set-new-date-for-crew-launch-explain-merlin-engine-issue/
"SpaceX technicians removed the two engines and shipped them from Florida to the company's test site in McGregor, Texas, where they were able to replicate the problem. They found that a relief valve within the gas generator—a tiny rocket within the engine that starts up and powers its machinery—was clogged with a masking lacquer akin to nail polish. They were able to show that removing the lacquer from the vent hole allowed the engines to start up normally.
This lacquer is applied during an anodizing process to treat aluminum components of the gas generator. It is supposed to be subsequently removed, but in the case of these two engines, a tiny amount of the material had been trapped within a bore hole less than 2mm across. "