Bentgrass and or crabgrass are problems in many lawns especially in lawns that are not maintained on regular schedule. Regular fertilizer application and lime can help keep desired grasses healthy and thick, test soil to see what lawn needs. Spraying lawn with Roundup will kill the Bentgrass but wait and seed at least 30 day's before seeding, the residual effect of Roundup will kill new seedlings. After 30 day's use a slit seeder and re-seed with good quality grass seed. Don't worry about thatch it will keep new seedlings moist and help lawn get started.