Flashback on masked plywood

I used masking tape to engrave on plywood and have discoloring around the design. Why is this happening and how can I clean it up?

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Just to be clear, you have removed the masking in this picture?

Tell us exactly what your process was, starting with having the bare plywood and the masking in hand.

Describe the way you lasered it and then any kind of treatment you did after the fact up to the point that you took this picture. Looking at it, it looks like you have some sort of leach from a liquid, did you ever touch this piece with liquid?

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It looks like glue melt to me. The kind you get from old school tan masking tape.

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I tried to use some 70% alcohol to remove the scorching.

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I used 6 “ masking tape on the plywood and lasered it with the engraving setting for light basswood plywood on my Glowforge Pro. I saw lots of flashback after removing the tape and tried wiping it with 70% isopropyl alcohol. Obviously, that didn’t work. Do you think that staining it will work? This is the back piece of the album with the tape removed.

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Yes, I used that kind of tape.

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The alcohol is what caused the color to get impregnated into your wood fibers. It dissolved the residue, and then got wicked into the wood fibers, premanently impregnating the wood with the color.

It sounds to me like you may not be using proper masking tape for the job and also that your technique for applying it may not be quite there. What you are seeing is what happens when the tape isn’t fully adhered to the material and there’s a small gap for the smoke to get under and stain your material.

You should take a look at these posts:

A tip on material prep:

Where to get proper medium-tack masking, #2 on this list:

On the importance of properly pressing down on masking:

My preferred tool for pressing the masking:

And lastly, I don’t believe you’re seeing flashback on your material in that living hinge cut, I think the masking is protecting it properly. If you’re wildly overpowering the cut you will flash back hard enough to cut through your rear masking and damage the surface; that looks undamaged, simply smoke-stained as in the case where your masking isn’t properly adhered. You certainly aren’t seeing flashback on the engraving, since that doesn’t even cut through, you’re never talking about the back of the material there anyway.

All might not be lost though, if you get some smoke stains on raw unfinished plywood like this my advice would be to lightly sand with ~220 grit sandpaper, the surface staining will come right off.

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Thank you

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I tried the blue 3m tape and it seems to take off a layer of the wood.

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As I said, #2 on that list points you to what’s known as “medium-tack” masking, it’s not as sticky as blue tape or standard masking tape. You want it to stick enough to seal the wood from the smoke, but not so much that it pulls up fibers. Scroll back up for the link.

They have low- and high-tack too. For most wood/cardstock/cardboard applications I find medium is a sweet spot. For hard plastics like acrylic, the medium works but sticks really well – to the point where it can be a pain to remove. I’d be tempted to try low-tack if I did a lot of acrylic work…

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Thank you. I will try this.

Better than masking acrylic, is to use @evansd2 method of using wood glue to cover your acrylic (I let it dry, so you need to plan ahead) if you’re engraving and want to paint it in. The glue peels off nicely and leaves sharp, clean edges. If you’re just cutting acrylic, wiping on a thin layer of dish soap works great and washes off easily, plus it doesn’t leave ridges on the cut lines like masking does. I always pull the masking off acrylic before cutting for that reason. And come to think of it, I wonder if you’re just using a thin layer of wood glue if you need to let it dry completely, as the dish soap isn’t dry. :thinking: Do you let your wood glue dry @evansd2?

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Here’s the post about glue masking hard surfaces:

Yes, definitely. My thinking is that it takes a lot of energy to drive off water. Cutting before letting the glue dry might impact the cut settings, not to mention that I don’t want to get wet glue in my Glowforge.

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Is there a specific brand of this tape? I looked on Amazon and couldn’t find it. Thank you for your help.

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Is this link not working? I’ve mentioned it twice now (three counting now), it has what you need:

It has links to other discussions, look at the one titled “cheapest 12” wide masking tape”. I also just added some bold headers to help people figure out what that post is showing you. Sorry if it wasn’t clear.

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My apologies. I recut the project and it came out better. Some flashback, but not as bad as before. Any suggestions for a top coat? I don’t want to damage the living hinge. Thank you. ( the shading beneath the tree root is supposed to be there.)

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Searching the forum for “finishing” will give you just what you need. Here’s what I found:

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Thank you

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Awesome, that’s my design from the Glowforge catalog. It looks great on your project!
A couple of different things. Staining on the engraved side of a project that was masked is often because the masking wasn’t scraped down hard enough and the edges came up. Masking adhesive is pressure activated and needs to be scraped down hard, until it’s nearly transparent. Glowforge does a great job with their masking.
If the stains are on the reverse side, this is probably flashback and is from the laser bouncing off of the crumb tray and hitting the bottom of the wood again. A couple of ways to deal with this are: lower the power or raise the speed, or lift the material off of the bed a little bit with spacers.

Great project!
Mike

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Wow! Thanks, Mike. I am in the process of coating it with “wipe on poly”. I hope my husband’s nephew and fiancée will like it. It is a perfect design. Thank you.

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