This if from a frequently asked questions section in Cleaning Management magazine and their Cleanlink web site.
Wicking is when a high volume of fluid stains a carpet and, incompletely removed from the under layers and pads of the carpet, re stains the carpets from below.
Usually this happens with a change in humidity. More humid conditions in the room re liquify the dried coffee, soda, etc and it comes back up to the top of the carpet.
Everything they say has some value, especially the repeated messages of :
– Immediately blot and preferably wet vacuum the offending liquid out of the carpet,
– Start extracting a high volume stain as soon as possible,
– Use many repeated passes with your extraction tool.
What they don’t say is that until all the remaining fluid residue is encapsulated or emulsified you won’t completely prevent the wicking.
Hence, you need to use a substantial amount of cleaning fluid – much more than you would on a normal extraction to get enough cleaning chemical up next to the remaining stain to fully capture it.
Another good technique I saw was the “funnel” technique.
You put down a circle of clear water encircling the area where the fluid residue is believed to sit in the lower layers of the carpet (in addition to the cleaning solution you put down over the actual area), then extract only the the much smaller area of the stain on the top of the carpet. What this purports to do is pull the water laterally thru the remaining soil and flush it to the center of the spot, then out into the extractor.