The model engineer is best served by three liquid fluxes depending on the parent materials.
The most common is CuPSol flux. It is used readily with tin-lead solders on copper, brass and mild steel.
Stainless steel and aluminium alloys require a more active flux. Use STAYCLEAN Stainless Steel grade or Aluminium grade as appropriate.
The residues of all are corrosive and should be removed, which is easily done with water.
All three fluxes are corrosive and should be kept in a cool dry area, well away from children and pets.
Regrettably they cannot be sent by post - only by carrier. Pre-order for exhibitions and save the carriage costs.