I did a quick search online and this is what I came across:
If the stain is grease, lipstick, or another cosmetic that is not water soluble, you can try using a handi-wipe on polyester(test it on an inside seam first to be sure it will not disturb the color of your gown) but NEVER on silk! On silk it is probably safer to camouflage spots with something white and relatively harmless such as baking soda, cornstarch, or baby powder. White-out or white shoe polish is tricky and is definitely NOT a good idea for use on silk.
Not sure if this helps, but I hope your big day goes well! :)