The Strongest and Most Durable RV Sealant Available. FlexRock Coatings FX-100 Sealant is stronger than single-component coatings, so it won’t crack or delaminate when thickly applied. And best of all, there's no maximum mil thickness. Use with Flex reinforcement seam tape or Reinforcement mat and create a tear and puncture-resistant surface more durable than your original roof. It's the perfect choice for any RV owner.