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Hello I will help you with your question,
This is "seam sealer" available in caulking tubes at autoparts and body supply stores.
You need something to keep the water out of the bolt hole where they will rust and seam sealer works well. YOu could use RTV but it does not clean up as nicely and if this is a resto project you want to get the sealer which can be used on body panels and may other places along the way.
Let me know how I can help you with this