Yes, you can cover your old lawn with 4 to 6 inches of topsoil and reseed - but grass roots will grow deeper and need less water if you first dig up old sod and work new topsoil into old soil. Your old weeds will die with no sunlight and both of the weeds you battle are easy to smother. The real question is why these weeds grow better than a lawn -mostly due to poor fertility of the soil. This means good topsoil will encourage a thick new lawn and a healthy lawn blocks out new weeds. A healthy lawn also needs sunlight, regular fertilizer and water during dry times. A healthy lawn is your best defense from any weed invasion. Keep growing, Marianne Binetti