Brent-
If you use good teak oil (Amazon's Golden Teak Oil or Cetol Marine Oil) you shouldn't have to refinish your platform all season. If your platform is in really bad shape, you need to make sure it is completely clean, sanded like you did, and absolutely dry. You should go to
www.Malibuboatowners.com and do a search for swim platforms or teak, they have great articles on preparation of your platform and application of the finish.