Plexiglas painting anomaly

[Xxray]

I know he'd be impressed with my CNC machine if nothing else !

Cleared the white today, unfortunately not perfect. Has some crackling around the word "die" and the crossed hammers. Not near as trashed as the 1st one though, but any imperfection really stands out on these. After the success of the 2nd one I cleaned the heck out of it before painting, I know I didn't miss any spots but apparently some contamination was left, weird. And thats if the film is the problem, would seem so, I wonder if drying time could be a factor ?

This is 3/8 plexi by the way, is 12x12", etch takes about 40 minutes [I know "etch" is not the proper term]. All done with a drag bit.
Glad I posted these pics, looking at them I noticed I need to do some node editing towards the top of the blade, didn't notice that before.

[glenninvb]

Did it crackle before the clear? and what's the clear for?

[LittleGreyMan]

Well, as clear PMMA (generic name of plexiglass) is not supposed to be painted, I guess the manufacturers don't care using products (in the chemical composition, in the process or in the factory) incompatible with painting.

So, depending on the manufacturer, the batch (immediately after machine cleaning or long time after), … your PMMA may be polluted or not by these chemical agents.

I'd try to clean the PMMA with different compatible products: dish washing detergent, alcohol, …and identify which is the best.

HTH

[Xxray]

I would imagine not many folks feel a need to paint plexiglas, and actually, it takes paint just fine - Its the reaction with the clearcoat thats the problem.

Crackling was explained in the OP but I'll rehash.
The clear is applied after the etch, and that is when it will crack. It is applied to restore transparency in the etch, which is left a bit on the opaque side.
Since my goal is to have contrasting colors, I need this transparency. I have tried painting without this step, it does show through but is not very vivid.

[Mobius]

I have come across the problem before, painting other substrates (I have never painted plexiglass/acrylic). It is either contamination on the surface or insufficient cure time on the previous surface. Since it is new paint, I would guess cure time.

For instance, I applied paint to a sign. The recommended cure time was three days, I waited 48 hours and was rewarded with the same issues clear coating it as you have. When I re-did the project and waited 4 days (to be safe) I had no issues.

The issue was that even though the paint appeared cured, it was still curing and off gassing in some areas where the paint had been applied thicker. This reacted with the clear coat.

[LittleGreyMan]

Another idea: you use the same combination of clear and painting on various materials without any problem, but did you make a test with the 2 rattle cans that produced the issue with PMMA? One of the products formulation may have changed. In this case, it will also occur on previously unaffected materials.

Mobius is right: it can also simply be a curing problem, due to specific atmospheric conditions. Temperature and relative hygrometry can have huge effects.

If it's a contamination issue, it can occur by migration through the first layer.

[Xxray]

PMMA is not a very active acronym here as far as I know, in fact first I have heard of it.

Everything I posted here was using the same 2 cans. I have plenty of semi scrap to experiment on but no real tests.
I have almost proven to myself that cleaning is a factor, I am going to go with more of that, maybe trying alcohol as suggested ,, Though that itself leaves a residue so I don't know. The 1st one with no cleaning was 90% trash, 2nd with cleaning was 100% good, 3rd one with same cleaning was 95% good ,,, So film/residue/contamination has got to be a factor, but cure can be as well.

I must admit, I generally do not follow proper cure times. Most things I do are on the spur of the moment or special orders so I don't have a whole lot of time to wait for proper drying.

[Xxray]

Here is the white, ouch ! So close to perfection, this would have been nice.
4 small spots cracked, enough to ruin the whole thing. I'm going to make a base for it anyhow and donate it to the hall, they can use it for a raffle giveaway maybe.

Am making progress though, almost convinced it is some form of contamination reacting with the clear coat - If it was a matter of curing time, why would it just do it in a few spots ?

[Mobius]

Thickness imperfection of the acrylic causing a low spot where the paint pools. Added thickness of the paint causing longer curing time.

Try one where you leave the paint to cure for a few days and see if that makes a difference.

[Xxray]

Thickness imperfection of the acrylic causing a low spot where the paint pools. Added thickness of the paint causing longer curing time.

Try one where you leave the paint to cure for a few days and see if that makes a difference.

Perhaps, sounds logical - But how would that explain the 1st one, that was trashed almost entirely ?

[Mobius]

For the first one the black paint probably wasn't cured enough.

When you sprayed on the first coat of clear it immediately reacted with the uncured black paint in a few spots, likely where it had 'pooled' and was less cured. The addition of the solvents present in the clear coat likely 'weakened' the bond of the black paint everywhere else that it eventually crinkled, however it was cured enough that it had little to no immediate effect with the first coat.

When you sprayed the second coat of clear, it doesn't just lay on top of the first coat. The solvents in the paint 'liquefy' the first coat, so that the two coats bond together. Because the solvents in the clear had weakened the bond of the black, it now crinkled when the second coat was applied.

If you had waited until the black was completely cured (as per the manufacturer specs, taking into account temperature and relative humidity) before spraying the first coat of clear, you likely would not have had a problem. This is assuming that the two paints (black and clear) are compatible.

[Xxray]

Dang, that is probably as good a hypothesis as we'll get.

I'll take your suggestion and try a 2 day cure and see what happens. As it stands, sometimes I let them cure over night, more often just a couple hours.

But that couple hours works just fine with other materials including but not limited to mdf, wood, metal and color core with the same paint/clear combo, why in your opinion is it set off with plexi ?

I'll add too, I have never used 2 coats of clear before, not necessary. I did it as a test and thought maybe a first coat would stop the crinkling after the etch. In fact, it seemed to have made it worse.
The drag bit scraped away all the paint obviously, so in the etched areas there was nothing for it to pool with, unless as residue.

[Mobius]

I'd like to add that your 'testing' hasn't exactly been scientific. Try sectioning off a scrap piece and then paint it black (or white, or do one of each). Then spray clear on each section according to a time schedule. Ie: wait 4 hours for the first section, 8 for the second, 24 for the third, 48 for the fourth etc.

If I'm right, the first couple should crinkle, but as the colored paint cures it should be reduced or eliminated all together.

Or... just follow the directions. They don't print them on the can just for shits and giggles

[Xxray]

Yes, quite obvious the testing "hasn't been scientific".
It is quite likely no one on this forum has any direct experience with this, and thats saying something. I've never broken out my magnifying glass to read the instructions of a rattle can, if it'll make you happy maybe I will - Or feel free to perform your own scientific tests and post back with the results.

[LittleGreyMan]

This is of no help, but PMMA (polymethyl methacrylate) is just the chemical name of this material. It seems in English acrylic is more commonly used, but it may refer to other products. Plexiglas or Perspex are trade marks.

Sorry if my post sounds harsh, no offense intended. My English is not polished enough to smooth things.

As Mobius noticed (BTW, with a smiley), except with a lot of luck, you probably won't be able to solve this without more "scientific" testing.

The problem seems tricky:
-you obviously mastered your process until now (and thanks for sharing it, this use of clear is very interesting)
-it seems nobody else on the forum uses it, so no practical experience to share
-at first sight, nothing has changed in the process and materials

It may also be something just under your nose and you do not see.

In such cases, I ask someone else advice. Not as you did on this forum with people who may have some knowledge. I just show the problem to somebody who is not aware of the job (an employee, my wife, one of my kids, a friend). I explain slowly, methodically and precisely the context, and the issue itself. Very often, the miracle occurs: while explaining in details, I realize which detail I didn't care of or I didn't see, or the newbies asks the right question or makes the right remark, putting me on the tracks.

If it doesn't work, you probably have no choice than using a more "scientific" testing. I'll add to Mobius suggestion another one: pick a piece of PMMA (sorry ) scrap which used to work and a piece which doesn't work and process them simultaneously.

Reading carefully this awful instructions in small characters on the rattle can (I hate them too) is not an option. You may discover something.

I also emphasize on hygrometry: if your clear is a PU monocomponent one, it polymerizes with atmospheric humidity, so much slower if air is dry. Dry air can also produce electrostatic issues which may cause the PMMA to repulse coatings.

Still no answer, just tracks.