You can think of whitewash as a bucket of thin white liquid and a wide brush. You go over a wall once and it turns bright white. What matters is how thin that layer is: it sits on top of whatever was there before. The old colour, the stains, the small cracks are all still underneath — you just cannot see them any more. Hold that picture of a thin white layer laid over something, and the word's other uses stop looking like separate words.
From the word’s history
The source records the noun as simply white (adjective) + wash (noun), and the verb as partly white + wash (verb) and partly from the noun. So the literal sense — a white thing you wash onto a surface — is what the word says on its face.
來源記載這個名詞就是 white（白，形容詞）加 wash（沖洗、刷上去的一層，名詞）組成；動詞一部分來自 white + wash（動詞），一部分來自這個名詞。也就是說，「刷上去的一層白」這個字面意思，就寫在字的表面上，不需要繞路。
Wiktionary entry 'whitewash', etymology section supplied in this packet (kaikki:3f6f69a190036ae4)
登入就可以看每一支怎麼連回核心畫面。