We add new liquid sealant to your tires to keep them sealing punctures,  recommend every 3-6 months by injecting it through the valve stem after removing the core, or by breaking the bead for a full clean-out and inspection. It involves removing the valve core, shaking and injecting sealant (30-100ml depending on the tire), reinstalling the core, pumping up the tire, and spinning it to distribute the liquid, ensuring continuous puncture protection. 
Price is per tire.