Question re old cards
 

I've noticed some old Exhibits cards have decent grading (PSA 3) for example, but have really unattractive brown splotches very prevalent.

Is it possible that mold or some other decay can start to rot a card after it is slabbed? Or a process in place continues after slabbing.

Just seem some ugly old browning cards that have decent registration.

It is reasonable to assume slabbing stops the aging process? Or can cards go to hell years after after slabbed?

No universal rule, as its a different situation for different cards. A holder won't stop damage. However, for many Pre-War cards, the damage is from a long time ago, and as long as the card has been dry in recent years and is dry now, it's likely okay. If the damage does appear to get worse and/or the mildew or whatever is from recent bad storage, then you can have the card treated-- there are methods to fix it. Keep the card dry and keep an eye on it. If the professional grader holders hold the card in acid free material (I don't know what they use, but assume they use good materials), that's a good thing for helping preserve the card.

Foxing, or those brown old age spots, are not uncommon on old ephemera. The good thing about it is it is good evidence the item is indeed old.

Discolored cards don't normally hurt the grade, so maybe you're just seeing a "flaw" in the card grading scenario. If there are stains, PSA labels them as such, unless they're small and the grade is below 4. However, they also have the discretion to reduce the grade if the submitted requests "No qualifiers."