Well, de-essing MEANS reducing selected fqs volume
However, try the following:
Monitor the de-essers side-chain, and emphasize offending fqs. Most de-essers have several controls 4 that. Try a peak @ 10Khz 4 s, and @ 6Khz 4 sh. Avoid shelving. Personally, I prefer the wideband mode 2 the HF only one.
If side chain access is available, try adding an additional eq unit and emphasizing offending fqs even more.
Use the look ahead feature if available. If not try advancing (negative delay) a copy of the track and feed it to the side-chain.
Adjust threshold 4 9 dB GR max.
If more, U might b working on the wrong fqs.
BTW: 18Khz is more of an air fq, not a cutting 1. U might b boosting 18k broadband, dragging along most harsh sibilance fqs. Trying boosting LESS in the right bands, or cutting wooly & muffled junk out.
De-essing can b achieved by inserting an eq in a compressors side chain as well.
If Ure in a DAW environment (I presume U r), U can:
1.Edit out the ss, ts & shs, dump the cuts onto a different track, and balance the 2. or-
2.Copy the track, emphasize offending fqs, gate, reverse phase and add 2 the original.
If compressing, try using a multi-band compressor like the C4 instead, and use a higher ratio on the HF band.