Does it het stiff?

In response to your very good question, and thanks for asking - I never thought of that either, but:
No, mine didn't stiffen up, I think if you were to use too much paint that may very well happen. The brand I used is "Decor Art"

I read the instructions on the medium bottle, and it says the following:

"that it softens and improves adhesion of acrylics and designs on textiles".

and further down it says:
"that once it air dries, you should heat set it with your iron for about 30 seconds, then you can wash on gentle cycle. Non toxic and water cleanup".

I trust this information will be to your assistance..try it and let us know how it works for you.

Happy Quilting

Amazing!