Renovating a sparse lawn
You can renovate a sparse or heavily thatched lawn. Just remove any weeds, aerate the soil, reseed using an appropriate seed mixture and topdress with compost. If more than 50% of the lawn is weeds, there is little organic material, the soil is very poorly drained or compacted to a depth of over 6 cm, it is better to remove the sod, work the soil deeply, amend the existing soil and reseed (or lay new sod).
A healthy and well kept lawn, as well as optimum soil conditions, will greatly stifle the spread of weeds. Ground that is too compact, too clayey, too acid or too alkaline supports weed growth, as do lawns mowed too short.
The use of weedkillers is generally not the best solution since they do not attack the cause. The ideal would rather be to pull the weeds out as soon as they appear, before the lawn is invaded. And then tackle the cause.