Just got this response - finally- from DYLON UK:
[color:"purple"] Thank you for your recent e-mail.
We do have fabric paint and pens but these are only suitable for washable fabric that must first be washed and dried. The product is then applied and left overnight to dry before heat sealing. This is usually with a hot iron and a cotton cloth although a hair dryer can be used.
I would imagine that the fabric has been treated in some way or another with a type of finish. This may be waterproofing, stain resistance, mould & mildew protection etc. Any type of finish prevents the fabric paints adhering properly and so would not hold when wet. For this reason I would not recommend using our products but of course you may wish to try a sample area of fabric to check for the results.
Mrs K Dyer
Consumer Advice
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What a great name for someone working for a dye company!
Despite her scepticism I'm going to give it a go on a piece of spare RipStop and see how it works out.
I'll let you know what happens......