iam going to give hypo ago I will buy some and experiment in my garden if iam not happy I will use it as a weed killer
Wear ppe, old cloths and keep pets, childrens out of the area you are working on until the surfice is dry and don't let any adults track hypo back into the house either.
Tips:
When mixing 14-15% strength hypo in a bucket 1-1 add the water first
Wet the area down first with a hose including any run off areas well like grass. plants etc and best you don't use hypo if it's near a pond as any run off can kill whatever living critters s in the pond.
Apply the hypo to the surfice libraly with a chepo car/truck telescopic brush (slap it on)
For bad areas I've found those stiff bristle deck brushes idea to give it a bit of a scrub if need be.
Let dwell for about 30 mins and re-apply if need be
Then rinse the area down well including any run off areas.
Job done