Wow. I've never had the stones to take on a mesh that dense and complicated. I'm also a newbie myself, but I have run into problems with artifacts like these. You have a TON of "Ngons" in there.
Trying to clean this up by editing the joined mesh, I think (and like I said, I'm a newbie and no real expert), isn't likely to end well. For one thing it'll take forever doing it by hand. For another, even if you have the patience to edit something like this by hand the results often just leave you with a ton of clean-up work where things that originally curved nicely suddenly turn into flat spots that actually make things look even worse than what you started with. And you can try editing the normals, face, by face, by face, until you're blue in the face, but the results aren't usually worth the effort.
Your nails look pretty good. All nice quads. But where the nails meet the flesh - all those faces desperately trying to be triangles with a half a dozen vertices along one edge... trying to clean that up by hand editing the mesh is just going to frustrate you to the point of giving up on it.
The thing is, people who sculpt characters deal with this all the time. So there are re-topology addons galore out there. Some for free, some for purchase. Most seem to work by allowing you to sort of shrinkwrap quads over your mesh, which does two things - first it allows you to dramatically reduce the number vertices in your mesh and there by dramatically reduce your render times per frame for when you finally start animating your model. Secondly if you intend to join the nails with the fingers, being able to move and/or merge vertices while keeping everything essentially shrinkwrapped to your original geometry is a big help.
I've watched, several re-topology addon demos on YouTube, even tried a few, and although they generally didn't help me with my own problems with artifacts, I suspect they'd work on a job like yours quite well.