Complex sentence: The coating is then washed in turpentine solutions to remove the unhardened bitumen and retouched in the tonal scale of the chosen color to strengthen or soften the tones as required.
Split into simpler sentences: The coating is then washed in turpentine solutions, removing the unhardened bitumen. It is then retouched in the tonal scale of the chosen color to strengthen or soften the tones as required.