1To add or drop a small amount of liquid onto a specific spot — 油をさす ('to oil a machine'), 目薬をさす ('to apply eye drops'). Kanji 注す applies but it's usually written in kana. Distinct from 差す ('to thrust / hold up / point').
自転車のチェーンに油をさしておいた。
I oiled the bike chain.
目薬をさしたら、すぐに目の疲れが取れた。
I put in some eye drops and the eye strain eased right away.
ドアの蝶番がきしむので、オイルをさしておこう。
The door hinges are squeaking — let me put some oil on them.
ミシンに専用の油をさすと滑らかに動く。
Sewing machines run smoothly once you oil them with the proper lubricant.
2To apply color or makeup to a face or surface in a focused spot — 口紅をさす ('to put on lipstick'), 紅をさす ('to apply rouge'), 色をさす ('to add a tint of color'). Kanji 点す applies but it's usually written in kana.
彼女は鏡の前で口紅をさしていた。
She was putting on lipstick in front of the mirror.
頬にほんのり紅をさすと、顔色がよく見える。
A light touch of rouge on the cheeks makes your complexion look brighter.
白いシャツに赤い小物で差し色をさすと、コーディネートが引き締まる。
Adding a pop of red to a white shirt with small accessories sharpens the whole outfit.