I know sign painters use MDO but I'm wondering if it is a long term solution to use for flat panels in exterior doors ?
( MDO: Medium Density Overlay which is some type of resin impregnated paper over exterior plywood) ( I Think !)

MDO is complete junk. I have no idea how it ever came into popularity. I have never seen it hold up well, even with an epoxy seal coat prior to painting. If you want a wood product, I use hydro tech marine plywood, or Exteria MDF if that works for you application.

We have MDO in exterior applications with merely enamel on all surfaces (even exposed edges) that have held up phenomenally for 10 years. It's not to say the finish doesn't need refreshing at this point but it's mind blowing how well it holds up. I have MDO that we used for fixtures that were tossed outside in damp wet alleys, no sun, swampy moldy wet, and the parts are crazy stable to this day. No core delam, faces are still intact.

In an application where the edges are heavily sealed and protected I can imagine when the stiff we've used would ever fail.

I have a raw piece of 3/4" paper 2 side MDO with no edgeband of any kind that I screwed to my gate at home to let the yard guys know to watch out for the 3 turtles in my back yard.
It has been there 7 years facing north in full exposure. I have to go out every year and rewrite the message but the mdo is as it was 7 years ago. Great stuff. I use a lot of it for applications that are exposed to moisture. Inside and out.

I've used it for flat panels in budget exterior doors. No issues at all. .

John

I thought MDO was my secret and no one else knew about it. For over 40 years it was used as sign board as it was the primary material for sign painters of all sorts. We used it for flat panels for shutters and exterior work.
The US made stuff is excellent. We got some Chinese stuff once and has to send it back. It was like potato chips. Could hardly get it thru the table saw.

Ive had good luck with it consistently but I have read comments like Matt's numerous times that its nothing more than junk fir core with paper face. I have always assumed the import issue or import core with paper face applied here. We have always been pretty particular about domestic core so it wouldnt surprise me if "some" MDO falls apart and some doesnt.

We've used it many times over the years for odd projects. Never had any delam issues. I'd pickup a couple of sheets at our local builders only supply. I'd guess one of those lifts lasted them 10 years. Maybe mdo gets better with age, like a fine wine.