Paving with extensive weed growth will need to be repaired. If you go about it the right way, you will end up with a weed-free paved surface that looks almost like new.
Repairing existing deposits with Dansand® block paving sand and Dansand® stone dust for paving.
Completely remove existing weeds from the joints or use an environmentally friendly herbicide on them. You will not be able to combat weeds that have taken root inside the joints or underneath the paving using new joint filler alone.
Clean out the joints down to at least 30 mm using a scraper, hard broom or high-pressure washer. Note that high-pressure washing may damage the surface and base of the paving.
Clean off the flagstones or blocks with water; they will usually have been soiled from cleaning out the old joint filler. Use an industrial vacuum cleaner to remove scratched up soil as necessary.
Leave the paving to dry out before filling the joints.
Sweep the joint filler into the joints, making alternate motions diagonally and directly into the joints to fill them as effectively as possible. Use a surface vibrator for best results. Use a spatula to check that you've filled the joints well; you should only be able to push the spatula a few millimetres into the joint if it has enough filler. If not, fill up the joints with Dansand® joint-filling sand or stone dust.
Then clean up the surface with a soft brush to fill the last joints in completely. Sweep up any excess material, and keep it in the sealed up bag and put it into a dry place for later refilling.
This gives you beautiful, light-coloured joints without weed growth. Fill joints up again as needed.