What is mere cardboard to the power of the Glowforge. But yeah, I can see that might be a problem.
What about a plain sheet as a backer, and then cut out just the area you want to engrave through on the cardboard. That leaves you a sort of frame around the area to be engraved and will still hold it flat. Doesn’t help much with the shiny though.
Matte spray?
Clean it off with acetone after, which won’t effect the mylar (if it is real mylar).
its corrugated so you wont get a consistent burn through plus fire
Matte spray is an idea – I might see if I can get away with a light soap solution or something similarly cheapjack. But it would have to be uniform – in the successful pieces I can see hints of the texture of the the blue tape I used for masking.
The lessons I’ve learned from this:
Enclose magnets in masking tape if you want a handle to pick them up by; otherwise if they’re in the field of view that the GF scans for final prep, errors may result.
Masking makes a real difference to the power levels you want for engraving and cutting thin materials (so you need to pick one masking material and stick with it)
Flatness is a big deal when working with sensitive materials
Oh, and the usual Make Only One Change At A Time.
Btw, a picture that illustrates what a millimeter or two can do in terms of cutting/engraving power.
The right side was being held down by a magnet, the left side was bowed up a bit. (After this test, I dialed the engrave power down by about half and the cut power up similarly – that line you see at the far left is a failed cut.)
Paul have you tried the Paper Adhesive Masking tape? (Comes in large rolls and might be easier to get off the mylar afterwards.) If you put it on the front and the back of the mylar in the sandwich that @jkopel suggested, it should work fine … I used it on thin fabric that tends to curl and it kept it flat and removed easily afterwards.
What are you planning to use the engraved mylar on? 
This might work well for holding it down:
https://community.glowforge.com/t/paper-fun-with-seklema-mat/8917?u=bill_laba&source_topic_id=8906&source_topic_id=9104Can you trick the Glowforge by just laying a mask on it for visibility and then removing it just prior to the engraving process?
Nerp. At least I don’t think so, unless you can open the lid between the time it’s ready to print and when you press the button (anyone tried this?) When you select “print” in the web app, the head does a final scan of your material, and the masking would have to be there for that step. (And taking it off would have to not affect position or focus)
On my list. I had held off buying it, but no longer 
There are a lot things I want to do with the mylar, mostly stencils, but this was to see if I could make bookmarks. And apparently I can.
I think he meant cereal box cardboard (thin layer), not corrugated cardboard. Or a person could try Kraft paper.
Can’t do that. The print will be cancelled. And at least on my Pre-Release unit it becomes confused enough that the design has to be reloaded before you proceed. Otherwise the infamous and self explanatory “Something went wrong” error will pop up.
How about this mirrored acrylic? https://www.inventables.com/technologies/first-surface-mirror-coated-acrylic
I was hoping to use some in a project.
Me too. I think it’s masked, but I don’t know for sure.
I do both - Cancel in the UI and lift the lid depending on how smelly the stuff I’m doing is. I don’t get the something-went-wrong error. Used both techniques earlier this week doing the engrave calibration. Maybe the fixed it and it’s not PRU machine-specific? Although I have done the lift the lid thing before but don’t recall if the next step was resending the same job.
Doing both might work. If you cancel the print in the UI and then lift the lid it’s probably not seen as an emergency stop. But was primarily answering the question of whether someone could lift the lid after the scanning and processing step, adjust the material, and then press the Print button. You can not. It cancels the entire print.
I just received an order of that acrylic yesterday. It’s beautiful stuff. Hope it cuts well.
That’s not what I meant. Sometimes I hit Cancel and sometimes I just lift the lid. Both work to stop the current print and I can resume. You’re right you can’t restart where it left off but you can re-do the job and not get an error (on my PRU). I did that on the engrave test whenever I noticed it was burning through. Then I setup the next row and hit Print again.
Maybe there is a difference in our machines unless you are talking about lifting the lid after the Print has begun? I’m specifically talking about lifting the lid before pressing the H/W print button but after the scanning and processing step has begun or completed.
That. 
I have done the latter (before hitting the glowing button but after the button started to glow). I don’t recall it causing an error but then I’m old
Not at all arguing the point. Machines and firmware may be different or we may be seeing the same thing. But we both agree that they can’t do what was in the original question.
Laser newbie here. What about just putting a heavy piece of paper over the mylar and cutting through the paper?
Yep. I was just wondering if there was a firmware difference - I wouldn’t expect that at this late date but it would be interesting to know.
(In that “huh” and move on sense…nothing actionable if I did know. More idle curiosity.)
