So this is what the joint looks like.
It's solid, but as you can see, the screws are exposed so they'll be rusting pretty soon. Also, the joint looks horrible. It's PT wood.
What can I buy to make it look nicer, and the same time, strengthen it?
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Is that a hand rail of some sort? I would not want PT wood on a hand rail, the chemicals used to treat it are dangerous for human skin contact. If it is a hand rail, I would get a better untreated wood that can cover over the entire arrangement, then use a safe stain or sealant on that new wood. Offset the joints so that they don't line up with these joints and you kill two birds with one stone.
That joint is not very strong and it wouldn’t take much to blow it out, replace the boards and use a half lap joint ( even a butt joint with wood that doesn’t have it Ends all chewed up would be better than that ) and use exterior grade deck screws that won’t rust, they are made to withstand the elements.