@brucecampbell Hi Bruce,
The 210D fabric offered here is 8.25 oz (about 4 oz before coating). Doubled up, it would be much stronger than a painted 8 oz fabric because unlike paint, the TPU adds quite a bit of strength.
Seattle Fabrics offers a 430D heat sealable nylon, but it’s actually lighter than my 210D fabric, at 8.2 oz, which just goes to show that denier (D) isn’t everything (see the Fabrics page for an explanation). Stay away from their non-heat sealable PU-coated fabrics, as the coatings are so thin they are not waterproof (I have samples).
I have tried coating fabrics with paint-on “flexible” polyurethane, but it cracks and separates from the fabric when folded, so it’s not suitable if you want a truly packable boat.
I think Extrim Textil in Germany might offer some heavier fabrics, but the cost of getting it to Alaska might be prohibitive.
Eventually I will add heavier fabrics to the shop, but these things take time to develop and I have learned to stop estimating timelines when dealing with manufacturers!