The short answer is no, you don't have to take all the rails off to apply sealer. However, it's important to note that applying sealer without preparing the surface properly may not be as effective as you'd like it to be.
Before you begin sealing, make sure the surface you're working with is clean and dry. If there's any dirt or debris on the surface, use a soft-bristled brush to remove it. You may also want to use a degreaser to remove any oil or grease that might be present.
Once the surface is clean, you can apply the sealer. PU18 is a good choice for this, as it's a plastic tube sealer that's designed to create a waterproof barrier. Simply apply the sealer to the areas you want to protect, using a caulking gun to make it easier to apply.
Be sure to follow the manufacturer's instructions when applying the sealer, and remember that it may take several hours (or even days) for the sealer to fully cure. Once it's cured, you'll have a waterproof barrier that will help protect your Swift Challenger 2002 from the elements for years to come.
In summary, sealing your Swift Challenger 2002 is a great idea for protecting it from the elements. While you don't have to take all the rails off to apply sealer, it's important to prepare the surface properly to ensure the best results. PU18 is a good choice for plastic tube sealer, and following the manufacturer's instructions will help ensure a successful sealing job.