Learn Spanish (Latin America): El esmalte negro sí que dejó el acabado arruinado. - The black enamel really did leave the finish ruined.

This is the resultative state periphrasis (using dejar + past participle). While arruinó simply states that the action happened ('it ruined the finish'), dejó el acabado arruinado focuses heavily on the physical state that was left behind ('it left the finish ruined'). It emphasizes the lasting consequence of the action rather than just the action itself.

What exactly does the phrase sí que add to this sentence?

Sí que is a fantastic conversational tool used to add heavy emphasis or confirmation, similar to saying 'really did' or 'sure did' in English. It is much stronger than just using sí on its own. It tells the listener that the speaker is strongly asserting this fact, often because someone doubted it beforehand or because the result is dramatic.

Since arruinado is a past participle, does its ending ever change in this kind of sentence?

Yes! When you use dejar + past participle, the participle functions exactly like an adjective describing the direct object. Because it describes el acabado (masculine, singular), it stays arruinado. If the enamel had ruined a feminine object, like the table (la mesa), the ending would change to match it: dejó la mesa arruinada.

I usually see esmalte used for nail polish. Is that what it means here?