There's not much you can do about sibilance. Trust me, I do voice over recordings every single day. I mean, if it's there, all you can do is not make it worse. Wrong compression and/or EQ settings can make it worse. Mic choice and placement as well. Sibilance is natural. Has to do with the person's teeth.
A mild use of de-essing can do wonders if you choose the frequency well and don't get more than 4-6dB of attenuation at a time. Avoid the de-esser to work all the time. Usually, the energy of the esses is anywhere between 4kHz and 8kHz.
I'm not familiar with Logic, but I'm sure it has level automation.
Find the offending esses, Select them accurately (very easy to "read" in a waveform) and reduce their level instantly by about 4dBs. This is what works best for me...

T.

sibiliance

Well, I would first assume that you are using a pop filter in front of the mic to help mitigate this, but that will only go so far. I know this sounds kind of crazy, but one trick I have used for heavy sibilance is to rubber band a no2 pencill around the outside so it sits right in the middle of the capsule. It helps disperse that energy a little. (wood also tends to absorb high frequncies). Send a post and let us know if it worked for you.

I have heard of the pencil and rubber band trick, but have not used it. Try it! I have also heard that the metal "logo/strip" horizontally, over the direct center of the Sennheiser 421, may be there for that very reason???

Also, like popping "p's", there may be some value in pointing the sibilance out to the vocalist and having them try to experiment with minimizing it themselves? Don't want to "traumatize" the vocalists, but, they may be able to help(And learn). Another help may be a brief(Maybe "free"?) chat with your local speech major - a college professor, mayhaps? May even be some "sibilance" reduction vocal techniques on the web or at the library, or, as the other poster points out - at the dentist..?

Indeed, let us know if you try any of this stuff!

TG

I was told the pencil trick and it works.

I was also told that if you hang the mic higher and point it slightly downwards the plosives and sibilance sort of slide under the mic... because the shape of the mouth pushes the air downwards.

This won't cure a false teeth whistle but is supposed to help loads with everything else.

Trying this one later.

Sibilance has been my nemesis in doing by spoken word recordings. What works for me is:
1. Find the freq. range where the sibilance is greatest. for me that is around 3.8khz to 4.1khz.
2. Try out all of the mics... in different positions. For me, the AEA R84 DJV (voicer over adapted) ribbon and the Lawson L47 (Gene Lawson tweaked for voice) work the best.
3. Position mic at chest level or experiment moving up or down from direct line-of-fire with teeth.
4. Teach vocal talent to find the mouth pattern that is most sibilant and unlearn the culprit.
5. Although I avoid de-essers, I have tried them all... Drawmer, DBX, SPL, Waves, etc. And the most transparent for me is the SPL as you can dial in a setting that is quite low and still remove the esses. DBX is second in line.

Those have worked best for me.
SG

Short of hunting it down and notching it out, my experience is to first raise and angle down the mic (usually does the trick).

If you are hesitant to switch your mic placement, go with the pencil and rubber band. Yes, it works! (Smile wryly at the talent when they give you that, "This guy is a f***ing Macgyver!" look.)

Always use a pop filter! (Although they really need to call them "Spit Shields", since that is what they are really protecting your mic from.)

In a dead booth, I've found that backing the talent off the mic a good 15-18 inches or so, 6 inches above the mouth, and positioning the mic diaphragm about 10-15 degrees away from the mouth can be a good start before de-essing.