The challenge is that there is a difference between katahito's suggestion of "uncoated" and your original "to be coated". "Uncoated" (or "unpainted", "untreated", etc.) refers to the current state of the item. "To be coated" refers to the intended future state of the item. It's not the same thing, and any other expression may remove that important information.
Meanwhile, "pre-coated" means the item has already had a coat applied, normally as a previous step in the manufacturing process. Again, not what you are trying to say.
I would not worry too much about repeating the phrase "products to be coated", especially in a description of the manufacturing process. With technical documentation, it's better to be precise than to write with style.
One option is to use a pronoun like "these", "these products", or "the products", if it's in the same context:
Products to be coated arrive on the assembly line and checked for defects. Those without defects are set in the coating machine, and then moved to the drying machine.