I spent two hours proofreading my article before publishing to ensure there were no spelling mistakes.

proofreading 是动词 proofread 的现在分词形式。proofread 由 proof（证明、校样）和 read（阅读）组成，意思是"校对、检查文本中的错误"。proofreading 指的是"进行校对"这个动作。