This product is usually used by greenkeepers and groundsmen, applied as a spray from a knapsack or by machine, and is basically just iron. Whilst it may help what little actual grass you've got growing to green up, and may make a temporary impact on some of the moss, turning it black, it won't affect any weeds growing. If your lawn is more than 50% weeds as well as moss, its not going to do much in terms of restoring your grass to lawn, and a high percentage of weeds to grass might mean you'd be better off stripping out the lawn in autumn and returfing instead. If the area is fairly shady though, moss will always be a problem ongoing. Instructions for use in the link below - be sure not to get any of it onto hard surfacing such as paving, because it will cause yellow/brown staining.

http://www.sherriffgrasscare.co.uk/products.aspx?pname=Ferromel-20&prodid=82

Thanks so much for your input