Sometimes we want to add an extra airtight layer where two or more pieces of fabric are joined together, so we coat the seam with a liquid sealant that cures into a flexible layer of plastic.
The best sealants I have found are the polyurethane-based sealants made by Gear Aid (formerly McNett) called Aquaseal+FD, Aquasure (more common outside of North America), and Seam Grip+WP. These products were rebranded fairly recently, so you may find old stock for sale called simply Aquaseal or Seam Grip – these are the same products in different packaging.
I typically choose Seam Grip+WP because you get 33% more in a tube compared to Aquaseal+FD and it is less viscous, so the same amount spreads thinner and covers more area.
Caution: As part of its rebranding, Gear Aid has given two very different products very similar names and packaging – Seam Grip+WP and Seam Grip+SIL. Do not use Seam Grip+SIL on the DIY Packraft fabrics, as it will easily peel off.
There are many other polyurethane sealants available, but none of the types I have tried work as well on the DIY Packraft fabrics as the Gear Aid products.
Note: none of these sealants should be used as glue for joining pieces of fabric together or for mounting things on your packraft. For that you will need an adhesive such as HH-66 or Stabond.