I seem to be getting some rust bubbles on my rear fender wells typical I guess.

Well my question is I looked under the wheel well on back side and there are 4 1 inch holes with no plugs in them both sides. I stuck my finger in side and could feel a lot of sand and dirt. Should there be plugs in these holes to keep dirt and water out and causing the rust from the inside?

I'm going to fix rust and blow out all the dirt in there and block holes with plugs. Is this a good Idea
Then going to put on black fender flairs to keep the rust from spreading.

thanks