Q:

Can I cover someone in Quintessence while time is frozen during Timestop?

After using the Time Stop spell, can I coat someone in Quintessence while they're frozen in time?

A:

Time Stop and Temporal Acceleration, that a psion is more likely to use, basically follow the same logic - while the time is stopped, other creatures are invulnerable to your attacks, spells and powers. The thing is, you kind of are not doing that.  
"Quintessence" is a substance that, if it fully coats an object, freezes it in time. The main problems with using it in any sort of battle scenario include:
1. The fact that one use of the power creates 1 ounce of Quintessence, that is enough to only cover the smallest objects (you'll have to practically get hundreds of castings of Quintessence to cover Medium-sized creatures, which is either expensive, if you're not the Psion doing it, or PP-consuming if you are)
2. The fact that quintessence is very gooey and thick (so it's harder to coat an object with than, say, water or oil)
3. The fact that you can get rid of Quintessence by scraping it away (any non-helpless creature resisting the coating will get rid of it faster than you could coat it)  
I assume that the first problem is already solved, hence this question. Does using Time Stop-like ability helps to circumvent second and third problems? Kinda, and yes.  
See, while you're in Time Stop, while the enemy doesn't resist anything you might do to them, the Time Stop itself kinda does it for them. In fact, you can not directly affect other creatures or their belongings:  
Time Stop text says:  

You cannot move or harm items held, carried, or worn by a creature stuck in normal time, but you can affect any item that is not in another creature’s possession. 

And Temporal Acceleration dictates:  

You can affect an unattended object but not an object held, carried, or worn by another creature. You are undetectable by any means while your temporal acceleration lasts. 

By coating another creature with Quintessence, we're practically putting it into creature's possession. Somewhere around the moment of impact of Quintessence with the creature or its other belongings, Quintessence becomes a part of the creature's belongings, and locks in time as well. This practically means that we can not spread jello-like time goo on our target, not until the time goes back to normal. We can cover it in dollop-like time jellos without spreading them, but it would be easy to miss a spot, and it would require a LOT more time jello to achieve that effect.  
Another way would be setting up an appropriately sized Quintessence "jello ball", hanged right above the target, so when the time unfreezes it falls on the target, locking it in stasis. Note that it would most likely allow for a Reflex save on the target's part.  
In short:
It would be an interesting trick, but the Time Stop itself makes it practically unusable. You can still use Time Stop to set up a Quintessence trap thou.

