[nickik]

The clean them pretty well. For example the clean along the welds to check them. The soot only stays on the parts where it doesn't really matter.

Engines are also cleaned. In fact, one landing failure happened because left over cleaning alcohol in the engine itself.

[latchkey]

"Small amount of isopropyl alcohol (cleaning fluid) was trapped in a sensor dead leg & ignited in flight"

https://twitter.com/elonmusk/status/1252985622219960327

[nickik]

Thanks for tracking it down, I was to lazy.