How to back paint clear plexiglass so I can reverse laser engrave?

I'm trying to back paint a piece of clear plexiglass so that I can reverse laser engrave. I'm then going to backlight the piece with an LED panel but I want the paint to be opaque so that the light only comes through the engraved pieces if held up to the back painted side. It will then have a nice look because the clear acrylic will give the lasered piece floating characteristics. I'm able to create this effect using black krylon fusion spray paint, but it just seems to take 3-4 coats in order to block the light out through the paint. I've tried black vinyl, block out sign vinyl and a variety of painted applications but nothing seems to work better than paint. The vinyl applications are also troublesome because the margin for error is too high with vinyl applications as I need the piece to be very consistent on the black reverse side. 

I've also found products out there specially made for reverse engraving, but they are extremely expensive, and upwards of 150$ for 24" - 48". I've talked to a couple people that recommended Krylon Fusion normal paint, I'm just worried that if I were to use a roller,  or brush, I'd see the brush strokes from the clear side and it would ruin the piece versus using spray paint, which is slightly difficult to get a consistent piece. 

Does anyone have any recommendations for what kind of coating or solution I would be able to use?


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gizmologist2 years ago

I've done this very successfully on my last project. The key caps on this object were painted with 4 coats of black Krylon, applied in a cross-hatch pattern. Just hold it up to the sunlight to check for thin spots. These were front painted so I could glue to the back, but principle's the same. Good luck!

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rickharris2 years ago

Spray paint it. Works fine.

Bdilla (author)  rickharris2 years ago

Ok thanks for the response. Overall do you think it's best to cross hatch the spray directions and rotate the piece of plexi-glass 90 degrees to get a more even finish?

As long as it is covered .

You can after engraving spray with a contrast cover to make the text stand out I will show an example tomporrow.

Bdilla (author)  rickharris2 years ago

Would the contrast cover also be translucent so that the light will help it shine as well? I just don't want to take away from the etched lines. I know if you use "cast acrylic" the etching creates a frosted look, versus being translucent like extruded acrylic would remain.

Bdilla (author)  Bdilla2 years ago

Hey - Just wanted to see if you could show me that picture? Please let me know and thanks again for the tips!

This is a sign I made for the local Allotment Association. It is laser engraved on mirror acrylic, The engraving was then painted on the reverse and after a coat of black paint was sprayed on top.

The technique works just as well with a layer of spray pain put onto clear acrylic as well. No visible frosting.

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Kiteman Bdilla2 years ago

If don't want the frosted look, I would mask off the areas you want clear rather than etching away the paint.

If the shape is complex, cover the whole sheet in masking tape, then laser-cut the outlines of the design into the tape. Peel off the parts you want black, leave the paint to dry completely, then peel of the rest of the tape.

(You may need to use an exacto blade to cut through the paint around the edge of the tape, so the paint doesn't get peeled off. Test the method on a scrap piece.)

Bdilla (author)  Kiteman2 years ago

Ok thanks for the response. Unfortunately the artwork is extremely complex therefore I would not be able to mask it off. The process can be done I just wanted to see if there was a coating process that might be easier. I've seen people silk screen the pieces also vs. spray painting.

yes, subsequent passes made at right angles will (ime) help control coat consistency

In my experience, to achieve a consistent, even coat of minimal thickness at opacity, light coats are best. They dry quickly, so you can perform multiple passes over the course of a day to complete the job.

To expound on what I mean...The first coat should not even come close to making the panel opaque. Nor after the second. Only after the 3rd or 4th should you see it beginning to close in. Possibly another few light dustings to seal the deal.

fyi: A desk fan with flow directed over the surface will speed the dry time.

PS> Why? Because for msot of us, we're not professional painters. Pros could probably do it in one. But again, we're not pros!