This work aimed at finding ways to improve the leaching resistance of Scots pine (Pinus sylvestris L.) wood impregnated with water soluble fire retardant (FR). Sapwood specimens of Scots pine (10 × 10 × 50 mm) were impregnated with aqueous solution of guanyl-urea phosphate (GUP)/boric acid (BA). Limiting oxygen index (LOI) revealed that treatment could improve the fire performance. At the same time, thermogravimetric analysis (TGA) illustrated increased thermal stability after the treatment. However, since the FR itself was not fixed within the wood cell wall, it was extracted during water leaching (EN 84), and the wood lost its fire retarding property. The resistance to leaching of FR from the treated wood can be primarily improved while maintaining high fire retarding performance and thermal stability of treated wood by mixing melamine-formaldehyde (MF) resin with GUP/BA before impregnation to the wood. To mix GUP/BA to MF solution, due to the acidic nature of GUP/BA, the condensation/polymerisation reaction would be accelerated in an undesired way even if the solution was adjusted to non-acidic by NaOH. The resulting solution would not penetrate deeply into the wood structure, whilst it would be difficult to re-use the FR solution. In order to avoid the reaction proceeding in an undesired way, introducing 0.5 wt% of pentaerythritol to the GUP/BA/MF solution can decrease the reaction rate. Additionally, it improved the weight percentage gain (WPG) and fire retarding performance, without significantly influencing the leaching resistance and thermal stability. Overall, it is suggested that such a treatment could be a suitable methodology for producing exterior-use fire-retardant pine wood.