@tozmo1, Thank you. I thought about that option as well. But that would make it too light. The barn-door hardware can take the weight of the plywood.

Can you make it a triptych out of plywood with bypass hardware? Each one would be under 4 feet.

But then it won’t look like a painting. I am trying to hide the fact that it’s a reach in closet behind it

In my (albeit limited) experience, most barn doors have a cross piece, either angled or horizontal across the middle as well as a frame around them.

As far as I know, lauan panels are the most thin and lightweight plywood panels you can get. You can get much thicker plywood that is usually used for flooring. You'll need to specify sanded/smooth -- and you'll still need to sand it yet again for it to be actually "finished" smooth in order to use it as a smooth finish surface for a painting.

A word of caution: If you do not attach boards or your lauan or plywood to other lauan or plywood (with its grain going the opposite direction) or to wood in order to help it hold its shape, your lauan/plywood could warp over time, especially if there is any moisture in the room such as from condensation from the windows and/or moisture from a nearby bath and/or kitchen (boiling water).

Does your barn door need to be a specific thickness to work with the hardware you have or intend to buy?

@suezbell, the sliding door needs to be 1 inch thick

Definitely not a pro. This is just food for thought:

If you're wanting one smooth plywood surface, then, because of the potential for warping, instead of one very thick piece of plywood, consider using two layers of plywood with the grain of the front layer being vertical and the grain on the back being horizontal -- or vise versa.

You'll need to sand even sanded plywood again ... they do need that extra sanding to be entirely smooth -- so.that would need to be done first.

https://www.homedepot.com/p/12mm-Sande-Plywood-1-2-in-Category-x-4-ft-x-8-ft-Actual-0-472-in-x-48-in-x-96-in-454532/203414055?mtc=Shopping-B-F_D21-G-D21-21_1_PLYWOOD-Generic-NA-Feed-LIA-NA-NA-&cm_mmc=Shopping-B-F_D21-G-D21-21_1_PLYWOOD-Generic-NA-Feed-LIA-NA-NA--71700000044150928-58700004609752639-92700041149959762&gclid=EAIaIQobChMIwf2qpfSV6gIVFQiICR2PrApEEAQYAiABEgJvYvD_BwE&gclsrc=aw.ds

BEFORE you cut your plywood, invest in some graph paper and draw the sheets of plywood to scale on the paper and cut the paper to see how you want the pieces of the two layers of plywood to fit. It well be that that you decide you want to create a smaller centered smooth piece for your art and create a larger frame above and below it. You might even cut the plywood to create one option on the front and another option on the back.

Would a 4' high 5' wide canvas be more practical than a vertically centered 4'x8? It may well depend upon what will be painted on your plywood canvas.

On the front and back you could put trim (boards or metal) around the edges and atop any seams that show to create your frame for your largest space for the art you want there.

Absent a perfect fit (or filler to hide the seams) if you decide on a 4'x8' vertical painting or try for a 5'x8' vertical painting, you may need a second pair of vertical trim pieces 6" in from the outer edge -- but if you do, you could consider that "mat" for your framed picture.

The Zar wood putty worked better that the others I tried to hide nail holes and imperfections in wood to be painted and in paneling. It worked for a smoother finish and didn't -- or at least hasn't yet -- dried out and crumbled the way another did. Others with more experience might have another recommendation.

It would probably be best to finish your plywood canvas each/both sides, including adding any glue or filler while the plywood canvass you're lying flat on the floor so you don't end up with it settling/running. If you intend to glue the plywood together or glue on any trim pieces (in addition to or in lieu of small finishing nails), you'll likely need clamps for around the edges to hold it until the glue sets so you don't have your layers of plywood trying to separate. (Watch about accidentally gluing a clamp to your project -- oops.)

The 1" edge of your plywood canvas for your art might be difficult to finish unless your side/edge trim pieces are angled molding wraps over/around the edge.

https://www.google.com/search?q=trim+piece+with+right+angle&client=firefox-b-1-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjbk4GP95XqAhVLlHIEHXahCiwQ_AUoAnoECAwQBA&biw=1366&bih=654

Whatever you decide, do let us know how your project turns out.