I have a Seal press too and I don't think the platen is aluminum, but more likely an iron casting or something like that. It's awfully heavy and it doesn't seem like aluminum would be the most suitable material to make the heavy, flat platen from. But I digress...
Anyway, totally agree with mdarnton on the solvent soak and the scotchbrite over steel wool suggestions to clean the stuck-on adhesive.
I would also strongly suggest getting some of that teflon (or silicone?) coated release paper for when you start mounting, to avoid the results you're trying to clean up now. I use a top and a bottom sheet on my press and with care you should be able to use these for a long time. I don't think I've ever had to change mine and I've mounted many hundreds of prints.
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