Processes for making weather resistant, slow-decaying, durable natural fiber/coir geotextiles produce geotextiles having flexibility, permeability, light weight and cost-effective characteristics. In this process an in situ chemical grafting using a mixture of Cashew Nut Shell Liquid and aminoalkyl trialkoxysilanes with cellulose was done followed by curing in presence of sunlight, UV light or heat. The developed product showed durability and strength more than that of natural fiber/fabric and retaining natural fiber/fabric/geotextiles characteristics. The geotextiles have delayed bio-deterioration having wider long-term end use/applications. This process of making durable geotextiles is eco-friendly and retains the desired characteristic.