Cradling large, rigid, synthetic supports

So sorry, your question seems to have slipped by me.  It is difficult to give an exact mathematical rule for the size of panel that requires backing. It is determined by the rigidity of the panel material, the thickness of the panel, the size of the panel, and the amount of paint or other materials that will be applied to the panel.
I am also hampered by a lack of experience using PMMA as a support for fine art so I cannot really give you a thoughtful answer. Perhaps others here can comment. My preliminary thoughts are below but I would really hope that others with more knowledge of the subject could add something more authoritative.
I would think that with PMMA panels, the most important protection would be to have it in some sort of frame (something like wood) which would absorb the impact rather than transmit the force to the panel, which could crack or break.
Finally, if used as a painting support, I would suggest really scuffing up the surface to provide some mechanical tooth for the paint adhesion. An acrylic dispersion ground would also be sensible, as it would have a natural affinity for the PMMA panel

I have forwarded this question to a few others who may have more experience with this product and its applicability for fine art.

Hi –
I am writing a revised answer as I completely misread the scale of the work and that changes my recommendations somewhat. I will leave my original response below this one, however, as I do feel it captures our thoughts if ever going large, say something like 4’x5′.
In terms of the sizes you mention, you could get away without crossbracing (which is generally frowned upon for other reasons – but that’s another topic) as both of these materials are unresponsive enough to environmental changes that the usual cause of warping – changes in humidity – is really a non-issue. However I think perimeter bracing would be a good idea to facilitate handling, framing, and hanging. And to provide a little more rigiridty.

Between the two options, I continue to like the composite panel. It has better rigidity and is lighter weight. Plus at this point it is such a common panel that confidence in its performance has been steadily improving. While acrylic sheeting is definitely durable, and has an excellent track record for stability, it remains just a very unusual choice and would see no advantage using it unless its transparency was important.  That said, if using either of them we would suggest using an acrylic dispersion ground. For both some surface scuffing would be recommended for maximum adhesion.
In terms of attaching a perimeter bracing, there is a listing of structural adhesives approved for Dibond – and which are likely good for other brands as well – that can be found on page 36 of this Fabrication Manual:
https://graphicdisplayusa.com/downloads/Dibond%20Fabrication%20Manual_May%202011.pdf
 Buy also consult with the manufacturer with of the composite panel you plan to use for their advice.
Acrylic panel would be more difficult and complicated to work with in this way So another reason to likely prefer the composite panel.

Hope that helps.

Sarah Sands, Senior Technical Specialist, Golden Artist Colors and Williamsburg Oils
 
My earlier response is below where I misunderstood the scale involved
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Hi – We do not have direct experience working with either of these materials at this scale, so what follows is just some general thoughts. In terms of thermal expansion, rigidity and weight, the composite panel really does seem to be the better choice. If you look through this brochure, which shares a lot of technical information and does some direct comparisons with other materials, you will get a good sense of its advantages (at least on paper):
http://media.alucobond.com/pdf/dibond/DIBOND_Broschuere_EN.pdf
And here is a sheet on Plexiglas with similar technical specs:
https://www.alro.com/dataPDF/Plastics/PlasticsBrochures/Brochure_PlexiglasG.pdf
Other reasons I like composite panels are the fact that they are used a lot for large scale pieces in the commercial graphics industry, including billboards, display graphics, and exterior signage, so there is a wealth of information on how it performs at that scale and how best to mount them. While acrylic sheeting is certainly durable, to get the same stiffness you would need to go 2-3x the thickness and it comes with a much higher thermal expansion compared to composites. Ultimately, acrylic sheets are just a much more unusual choice at those sizes, with perhaps the sole advantage being perhaps its transparency, if that was important.
In terms of sources for advice, I would reach out to the manufacturer or major distributor/retailer for these panels as they would certainly have experience of how these perform at that scale. And if you were willing to purchase a panel at that scale, you might take a look at Simon Lui, who fabricates large scale panels for a lot of artists in NY:
https://simonliuinc.com/art-panels/dibond-art-panels/
He has a mechanical engineering background to boot, so comes to supports with a wealth of knowledge about materials and how they perform. And even just looking at his designs for integrating the panel with wood framing running along the sides can give ideas.
In terms of acrylic sheeting, it can be primed with acrylic dispersion grounds, as Brian mentioned, and unless you truly need the transparency we would recommend that.  For more information, also avail yourself of the resources from someone like Plexiglas
https://www.plexiglas.com/en/acrylic-sheet/
For both acrylic sheeting and composite panel you are working with very engineered commercial materials and the companies that make these really know the most about them – physical tolerances, how best to mount, the ability to span a large area unsupported, etc. Hope that helps. I can also say that we have worked with a lot more artists doing large scale pieces on composite panels then on acrylic sheeting and that alone says something.
 

​Silly me – I will be editing my initial response as for some inexplicable reason  I totally  misread the size. By a lot! Anyway, you will see edits in a moment. Thanks.

​Just to add a couple of thoughts, two points that Sarah mentioned caught my attention: the issue of stiffness, and whether transparency is important. I think it’s likely that any cradle or chassis behind a polymer panel of this thickness will cause some planar irregularity on the face. This is probably within tolerances for signage, but probably not for artistic painting. Also, it occurred to me that, if transparency/translucency is not important, the better choice would be to laminate the polymer to a rigid substrate. But, if one were going to do that, why not just paint on the substrate? 

If the use of this specific panel material is important, I think it might be worth trying a shallow box construction, two panels enclosed on all 4 sides with a sealed cavity of dead space inside. The sides could be metal, proud on the face to protect the panel edges. Kind of like a double pane window. This is just a brainstorm, however.