After reading the replies, that's what I'm going to do!
(thank you everyone for the info)

Such a good idea. Why didn't I think of that? I need to do that and then the back of my unmarked mat will be usable to me.

Thanks for posting this!

Great idea! Love the idea of adding some lines for straight edge too.

Super Idea, Does the permanent (as permanent as possible) rub off the mat and onto the fabric. I am a person that totally depends on those 1/8" lines for straight! I trust me, and I had better after 76 years, but my hands don't listen to me all the time, and the eyes are getting a little sassy, too! But I have, I'll bet a nickel to a rotten donut, at least 8 mats, all different sizes that are like brand new on the flip side. And mats are expensive. Thanks for the info,. That is something I am going to do. Edie

Love this thread....because just a month ago the light bulb in my "cheapy" brain went off and I started using the back side of my mats! Now you all are confirming my doing that...like the idea of a couple lines drawn on it too...

All these years and never thought about using the back of the mat.

My last mat that wore out, I just flipped it to the shiny side, then sanded it gently, and then drew straight lines with permanent ink so I could still line up my fabric. Worked great.

You paid for both sides of your cutting mat, so why not use the back. I never use the lines on the mat anyhow.
Sharon in Texas

I paid for both sides of my cutting mats, and use them both ways. If I am doing some smaller glue basting (blocks) I always use the back and the dried glue just peels right off