Here is some good advice about grading/conserving silver pandas . . .
don't do it
I am not joking
For many reasons, seriously I wouldn't. Here are just a few:
1) silver pandas are notorious for developing spots and exposure to air encourages this process when it otherwise might not happen.
2) the chances of getting 69 grades, the only grades that justify the cost of grading, are very hard to get with silver panda coins; most coins are 68s because of the double seal rattle effect
3) If a silver panda master set ever materializes, your OMP silver pandas will be valued just as much as 68 or 69 grades to master set accumulaters (if history is any guide). Therefore, no need to grade them!
4) You can spend that hard-earner grading money on nice new coins by buying undervalued silver pandad that are already graded. 69 grade coins from 1991-1999 are about 30 to 80% lower cost now than they were 3-4 years ago, which means they can be picked up on the cheap and sold for a profit during the next upswing
If these 4 reasons don't convince you, may God help you :)
Good luck with your decision.