Why things are hard to clean
Not what to use — why the last thing you tried did not work. Sixteen mechanisms that between them account for most cleaning that fails.
What this area turns on
Almost every stubborn deposit is stubborn for a nameable reason, and the reason usually means the deposit is no longer what it was. Burnt-on food is not strong grease, it is carbon, and no degreaser addresses it. A tacky old remote control is not dirty — it is the plastic's own additive arriving at the surface. Limescale did not settle on the tap, it grew there. Read that way, the question stops being which bottle and becomes which mechanism, and the mechanism decides the bottle.
The records
Adhesion and cohesion
ScienceTwo different forces with one outcome. Whether a deposit holds to the surface or holds to itself decides whether you lift it off in one piece or in a hundred.
Surface roughness
ScienceAt the scale that matters, most surfaces are landscapes. Roughness multiplies contact area, hides soil from a flat cloth, and turns wiping into redistribution.
Why old stains are harder
ScienceTime is not neutral. A stain that has sat is chemically a different problem from the one that was spilled, and four separate processes are responsible.
Heat-set stains
ScienceHeat does not remove some stains, it installs them. Three different mechanisms are involved, and they cover most of what comes out of a tumble dryer permanent.
Carbonisation
SciencePast a certain temperature, food stops being food. Carbonised residue is not a strong version of grease, and no degreaser addresses it.
Pigment retention
ScienceRemoving the substance and removing the colour are two jobs. A great many stains are cleaned successfully and remain visible, and that is not a failure of the cleaning.
Plasticizer migration
ScienceThe tacky film on an old remote, torch or camera grip is not dirt. It is the plastic's own additive arriving at the surface, which is why cleaning it off works briefly.
Crystallisation
ScienceScale does not settle onto a tap, it grows there. A crystal nucleated on a surface is bonded into it, which is why wiping was never going to work.
Penetration into fibres
ScienceA textile is not a surface, it is a bundle of tubes. A liquid on it travels sideways and downwards at once, which is why rubbing a carpet mark makes it bigger.
Why oil resists water
ScienceWater molecules attract each other so strongly that they exclude oil rather than dissolving it. Nothing about scrubbing changes that, which is what a detergent is for.
Why soot is tenacious
ScienceSoot is an extremely fine carbon particle carrying an oily fraction, deposited hot onto a surface. Every one of those three properties works against removal.
Whether a surface attracts or repels water decides what soils it, how it soils, and whether the cleaner you apply ever touches it.
Coating thickness
ScienceA great many surfaces are a thin layer over something else entirely. Once you know how thin, most compatibility questions answer themselves.
Two surfaces in the same room accumulate completely different soil, and the pattern is predictable rather than random.
Organic load
ScienceA disinfectant applied to a dirty surface is spent on the dirt. This is why every professional protocol cleans first and disinfects second, in that order.
Skin is not a surface
ScienceThe thing that dissolves a contaminant off an object dissolves the barrier that keeps your skin working. Same chemistry, entirely different consequence.