You can iron rayon with a cool iron only if indicated on the garments ‘care label'.
More Info: It is important with rayon garments to strictly adhere to the ‘care label' guidelines because there are a variety of rayon manufacturing processes and a variety of treatments added to rayon garments, each with its own care needs.
How to Iron Rayon
If the garment's care label specifies that ironing is appropriate, it's best to iron the garment before the deep wrinkles set it while it is still damp. Using a cool iron setting, turn the garment inside out to remove the wrinkles. If it does not remove them fully and you must turn the garment right-side-out, use a press cloth between the iron and the garment.
If you need to iron the garment between washings, dampen the garment with a light mist of water and apply the same method as you would for the damp garment.
Hints for Ironing a Damp Garment
To iron a damp garment, first roll the garment in a soft, absorbent towel and press out any excess water. Lay the damp garment on an ironing board and carefully reshape the garment before you iron it so as not to stretch it. If you have a large garment, such as a dress, keep portions of it rolled up in the towel as you are ironing so it doesn't become dry from air exposure. If the garment begins to dry, lightly spray the garment with water-use bottled water because tap water can contain minerals that can damage fabrics.