Nitrates are used as preservatives in food manufacture.

硝酸盐在食品加工业中被用作防腐剂。

Though the panels come pre-treated, they needed a final sealing with two coats of preservative.

虽然这些镶板运来时经过了预处理，但是还需要涂上两层防腐剂进行最后的密封。