“After dyeing, the fabric is finally treated with a durable water repellency finish (DWR) on both sides of the fabric for added performance and water resistance.”
“Restoring Water Repellency
Through hard wear and tear, Ventile
garments may eventually require a reproof. We advise that you follow the specific instructions on the waterproofing products we’ve suggested below:
Granger Extreme Waterproofing for Naturals
Nikwax Tech Wash or pure soap (ensure there is no residue of detergent in your washing machine)
Nikwax Cotton Proof”
From
https://ventile.co.uk/fabric/
If I understand correctly, this pertains to the UK originated, currently Swiss/Austrian ( Stotz & Co. AG)made Ventile version. Various treatments are applied to Ventile it depends on the manufacturer. The Japanese versions of ventile may be different but it’s harder to research.