Probable if u dont know the nature of varnish, I would suggest to make some spot tests on samples u create alone and place….3…4..5 different types of spray -varnish to see what happens on the surface, after they dry. Try to use same thickness, quantity and close quality to see how materials react. Varnishes used today on acrylics are mostly removable and also acrylic nature (so i dont know if the artpiece is contemporary etc). How u can see the embrittlement and stiffness? U have specific samples? Since acrylics arent so hard material as oils and cotton is also soft material. The varnish or coat that can harden probable such materials may be of oily ingedients,resins, as oil coating.