UV mapping went weird...

Hello there, chaps and chappettes!
I was creating a new mesh for a game which I’m making, but I ran into a slight problem when I was trying to texture it! When one is UV mapping an object, one should mark a seam in the appropriate place, yes? Well, the problem is that this mesh is very large and complicated, and to go around the whole thing thinking about how to unfold it, would take years! So, I pressed ‘a’ in edit mode, to select the whole object, and the pressed mark seam. I thought that this would work, the texture would just look a bit odd. I almost works, but whenever I paint onto it, other edges of the mesh also somehow get covered. For example, if i were to paint a few faces purple, I would end up with a few very very small, but still noticeable purple lines and dots spread around the whole of the rest of the mesh.

Is there an easy way around this?

Select all faces (A), smart unwrap (U / Smart UV Project)

Thank you so much!!! I haven’t been able to fix this for ages! like years! And it was so simple!!! It works! Thanks, man!! May the almighty lord notch, god of minecraft bless your soul!