Damp proofing liquid can refer to at least two things ..
Injection fluid, which is silicon dissolved in paraffin which is injected under pressure into bricks to become a damp block.
Paint-on fluid (again usually silicon based) to stop the surface of bricks etc letting water through - this is rarely a proper fix.
The four main causes of damp on internal walls (not in order) are ..
Damage to external pointing/rendering creating water traps which migrates through the walls
Excessive water on outside walls which can be due to leaking gutters/downspouts or splashing from another surface.
Risng damp due to the existing damproof course being bridged (by plaster, skirting board, external rendering etc)or damaged.
Condensation ... There is a lot of water vapour created inside a house from people, baths/showers, clothes drying, washing machines etc. This water vapour will condense on cold surfaces such as outside walls, windows etc. The cure for this is either a de-humidifier or ventilation. A lot of houses have been virtuallly sealed with double-glazing, chimneys blocked up, draft-strips etc, the house needs ventilation to reduce the humidity inside.
The trick to any cure is to find the reason.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates