I have produced a sheet to inform customers about filtration lines here are the words ...
Filtration lines information sheet
How they form
Filtration lines occur no matter the type of home or building.
Sadly even if a carpet has been fitted correctly & even to BS5253 draught marking may still occour. Since everyone now (including your client) demands light & plain carpets it has become commonplace.
As everyone knows, warm air moves up, and as it cools, drops back down. That's why hot air balloons defy gravity.
Something that not everyone knows is that warm air also likes cool surfaces.
This means that as air is warmed, typically while the heating runs, the air - full of pollutants - goes up and then searches out cooler surfaces, such as an outside wall or another room.
As the air moves toward the cooler area, it will push its way through carpeted surfaces (under doors, etc) and "clean itself". Just as carpet is a filter, it is much more so when air moves from one room to another, or when air moves under furniture, etc. What is left behind is a buildup of oily soils that are very difficult to remove. As more time passes, more soils build up and soon you have a tremendous cleaning challenge.
Their makeup is oily and the molecules of the soils are so small that cleaning them or removing them from fibres is difficult. In addition, they have an electrical "charge" that bonds them firmly to the fibre.
Challenges
Probably the biggest challenge to removing soil filtration lines is where they are on the carpet. Although about one-third of soil filtration lines are under doors kept closed most of the time, the other two-thirds are in areas very difficult to work on.
If your customer or client has a room that is seldom used, and the door is kept closed, the air that goes into or out of that room moves under the door - which means that the pollutants in the air are trapped in the carpet under the door.
After all, carpet is a filter, and when you concentrate air movement in an area (such as under a door) you get more soiling in that area.
You need agitation
The key to removing filtration soiling is agitation. Even the strongest chemical won't do the job by itself. Many carpet cleaners have attempted soil filtration removal by applying the correct chemistry and "scrubbing" the chemistry into the carpet.
These same carpet cleaners have found that they damaged the baseboards, walls or other close surfaces at the same time.
Many filtration soils are along walls that go up or down a set of staircase, or along the baseboards of walls. Imagine the difficulty attempting proper agitation against walls that you do not want to harm.
There are many good chemical formulations for soil filtration removal. Check with your supplier for the best ones for you. No matter which one you choose, be prepared for heavy agitation on filtration soils.
Better scenarios
By now, you are wondering why you would want to attempt removing filtration soiling.
Here are some additional steps you can take, after regular extraction techniques fail.
1. With a flip-top bottle, apply a gel solvent to the filtration lines (the use of a gel solvent is safer as it doesn't have a high risk of creating delamination)
2. To protect the wall or baseboard, use a "shim" of some kind - this can be a piece of wood, plastic, or cardboard that will protect the wall or baseboard
3. Work solution with as much agitation you can use with the particular fibre you are cleaning (a bone or plastic spatula or small brush works fine)
4. Before rinsing, apply a strong solution of hydrogen peroxide, always remembering that natural fibres need special care (for synthetic fibres, accelerate the peroxide solution with a few drops of ammonia)
5. Agitate the chemicals on the fibre and let them mix and dwell for several minutes
6. Rinse, but keep the shim close to the wall or baseboard, in order to avoid harming the wall or baseboard
7. Repeat as necessary, allowing the peroxide solution more time for a better chemical reaction