Plexiglas painting anomaly

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Plexiglas painting anomaly

Postby Xxray » Mon Aug 31, 2015 4:18 am

Here is the scenario.

I like to take clear plexiglass, spray paint it [usually black].
Then I do an etching into it, spray clear coat on it, then paint the etching another color [usually white]. So far so good, but the problems start with the clear coat.
Why even spray clear coat, you might ask ? Reason is simple - The etch leaves the clear plexi looking foggy, not clear at all. Spraying clear coat on it makes it transparent again, ready to receive the contrasting color. So this step is necessary, not optional.

Problem is, spraying the clear coat on painted plexiglas often causes it to crackle badly, this posted example done today is extreme.
First pics shows when I sprayed some clear coat on the black before etch, it crackled just a bit. After the etching and another coat of clear, the crackles went wild, ruining the whole thing. Last pic shows what it looks like from the front, only a very few areas are unaffected, almost like a virus.

I have run into this before, I started doing these designs a couple years ago it happened to be in cold weather, when it happened I thought it was temp related.
Does it just the same in hot weather too though, sometimes even worse.
I have painted, etched/cut then clear coated 1,000's of other things made out of MDF, color core, metal, not once has this happened so it is unique to plexiglas.
Odd thing too, I have done the very same thing and sometimes it never happens, comes out perfect. Can't post examples as they are not forum friendly.

I doubt if anyone will have a solution as I doubt there are many who do this type of thing, just throwing it out there in case anyone could recommend a solution.
This peice here would have been jaw dropping and could easily have sold for $100 if not for the crackles, now it is scrap.
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Re: Plexiglas painting anomaly

Postby FixitMike » Mon Aug 31, 2015 6:31 am

I would suspect the solvents in the paints. Have you tried using water based paints? Minwax Polycrylic clear is water based, although it does have alcohol in it. http://smile.amazon.com/Minwax-23333-Po ... TKDZRQFQYP
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Re: Plexiglas painting anomaly

Postby Xxray » Mon Aug 31, 2015 6:41 am

Preferably rattle can spray paint
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Re: Plexiglas painting anomaly

Postby zeeway » Mon Aug 31, 2015 11:35 am

Presume it is the black crackling and not the underlying plex? Some kinds of acrylic can do that when sprayed over oil based paint. I guess you are using the same brand/type of spray for both layers? Aren't there some sprays made specifically for plastics?

Sounds like time for some experimentation...

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Re: Plexiglas painting anomaly

Postby garylmast » Mon Aug 31, 2015 12:45 pm

There is material on the market that may work. I got my supplies from http://www.signwarehouse.com that I use with my Laser engraver, but may work on a cnc. A picture I did prior to a Model A convention I went to in Springfield, MO.
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Re: Plexiglas painting anomaly

Postby Xxray » Mon Aug 31, 2015 4:52 pm

Yes, black paint cracking and not the plexi.
I use rustoleum and it says "for plastics" right on the front.
What baffles me is why it does this on plexi and the very same paint/clear coat combo will not do a thing on mdf, metal or color core. As I said, I have painted 100's of mdf objects and clear coated them with the same stuff, never had a single crackle.

Could it be the invisible film left on the plexi from the peel off backing ? I've never cleaned new plexi as it looks clean enough when the backing is removed, but I'm going to give it a shot. It can't be reacting with the plexi itself, but something on it.
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Re: Plexiglas painting anomaly

Postby antennasDIY » Mon Aug 31, 2015 4:57 pm

Maybe it's because on wood the clear coat can soak into something, on plexi it sits and ruins the paint. I was using Krylon clear coat over a mask to seal the cut letters. I found it was shrinking the mask and maiking the resulting paint job look bad as the paint would leak under the now deformed mask. It was reacting to it. Try using a polyurethane for sealing?
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Re: Plexiglas painting anomaly

Postby Xxray » Mon Aug 31, 2015 5:10 pm

I'll try anything, but sealing is not the reason I spray the clear coat on. After all, the only thing visible when its turned around is the 1st coat of black, nothing on top of it. But what you say seems to have logic ,,, but, I have used the same paint/clear coat combo on metal as well and its never done this. Did one just the other day using metal flashing glued to mdf, painted black [2 coats], clear coated then etched, came out fine.

As can be seen in the pics, it reacted a bit just on the paint before it was etched, but it went crazy when it got to areas that were etched.
It would seem that the clear is seeping under the black, as it has a tenuous bond, and forcing it to lose what adhesion it has.
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Re: Plexiglas painting anomaly

Postby Xxray » Mon Aug 31, 2015 7:44 pm

This is what I use BTW - Ironically, alot of folks actually try for "distressed" look like this, if I can figure out how to control it from nothing to a little to alot, that would be great. Right now, "nothing" is what I'm shooting for, "alot" is what I'm getting.

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Re: Plexiglas painting anomaly

Postby Ms Wolffie » Tue Sep 01, 2015 12:18 am

As far as I know, plastic has a different chemical composition to what you call plexiglass (we call it perspex).
Have you tried on both extruded and cast perspex?
I tried some clear, that was touted as suitable for both metals and plastic, on aluminium.
It was even supposed to be acting as an etching primer.
After a couple of days it peeled off like the skin on a sunburnt back.
I have yet to find a paint that bonds to plastic, it just sits on the surface.
If your paint has 2 different chemical compositions, they will not bond but instead the drying ingredient in the top paint will shrink and crackle.
I have some crackle paint and the base- and topcoat have different ingredients in them.
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Re: Plexiglas painting anomaly

Postby Ms Wolffie » Tue Sep 01, 2015 12:19 am

As far as I know, plastic has a different chemical composition to what you call plexiglass (we call it perspex).
Have you tried on both extruded and cast perspex?
I tried some clear, that was touted as suitable for both metals and plastic, on aluminium.
It was even supposed to be acting as an etching primer.
After a couple of days it peeled off like the skin on a sunburnt back.
I have yet to find a paint that bonds to plastic, it just sits on the surface.
If your paint has 2 different chemical compositions, they will not bond but instead the strongest drying ingredient in the paint will shrink and crackle the other one.
I have some crackle paint and the base- and topcoat have different ingredients in them.
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Re: Plexiglas painting anomaly

Postby Xxray » Tue Sep 01, 2015 1:44 am

In theory you'd have to scuff up plastic to get a good bond, that seems logical. Don't want to do that with clear though.
Peeling is not my problem, I can get very good detail without it peeling off.
Its not the paint per se, as stated I have done 100's if not 1,000's of projects using the same paint with other materials and it never crackles, only with plexi.
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Re: Plexiglas painting anomaly

Postby Ms Wolffie » Tue Sep 01, 2015 3:14 am

Exactly what I mean.
There may be a chemical reaction between the perspex and the paint.
I believe you mean Abnormally?
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Re: Plexiglas painting anomaly

Postby Xxray » Tue Sep 01, 2015 4:01 am

No, anomaly fits the bill - "something that deviates from what is standard, normal, or expected."

Anyhow, tried another today using my theory that the film left behind after peeling the mask may be to blame. I used some "dirt away" and a soft rag to clean the virgin plexi, then spray painted it black with the same I used before. Let it dry about an hour and sprayed it again. Did the etch and I nervously spayed it with clear coat - No wrinkles, not even 1. Came out great, I'm not sure this solves my problem or what, I'd be thrilled if it does.
I just made another, this time using white as the background color and black for the fill. I did the etch but haven't clear coated it yet, white seems to take longer to dry than black and its still a bit tacky so I'm waiting over night to clear coat it.

Pictured below is my process. Etch is done and it gets a clear coat, unlike yesterday, no cracks. When it is going to crackle it happens almost instantly, I knew right away this one was fine. When that is dry it gets painted with the fill color, white in this case. Then back to black - This is purely for looks, it looks better black if viewed from behind ... Also, gets people wondering how the heck the white got in there.

I'll update when I see how the white one turned out, I'm going to edge light the black one. Edge lighting isn't as near impressive with painted plexi as clear, but adds a nice accent.
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Re: Plexiglas painting anomaly

Postby zeeway » Tue Sep 01, 2015 11:18 am

Thomas Edison would have been proud of you...

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