rosin spot

例句：

1. 1.The old painting had a rosin spot on the back.
   那幅旧画背面有一个油斑。
2. 2. The sun's rays hit the sand and left a rosin spot.
   阳光照射在沙滩上，留下了一个油斑。
3. 3. The artist carefully rubbed the paint off with a rag to remove the rosin spot.
   艺术家用布仔细擦去颜料上的油斑。