To give your new lawn the best possible start, prepare the area you are going to be hydroseeding. For proper growth, you’ll need 3” or more of topsoil. Any hard-packed dirt should be harrowed or broken up, weeds should be sprayed several weeks before application, and you should ensure the land is graded to spec.
It’s best to rent a sod-cutter, remove all the old stuff, and start from scratch. When attempting to rejuvenate patchy subpar lawns, we overseed with a light mulch fertilizer and seed mixture over topsoil – you’ll just need to cut it fairly short first.