Tecnics 1200G platter cleaning

Good question.  Without knowing more, I would just use a damp microfiber towel, rung out as far as it could go.  You don't want to use any chemicals that could stain the finish.  I don't know why the platter is getting dirty.  Dusty perhaps, but dirty?  

I have a boston audio mat on it.  maybe its carbon graphite dust, not sure

"One man's dirt is another man's patina."I would just wipe off any dust and let the brass age gracefully.

Try this: https://www.flitz.com/brass-copper-tarnish-remover/