PLA longevity?

Would a coat of good quality paint seal the pores from the air and make the PLA last longer?

It's not going to break down like that. I'm not saying that over time PLA and many plastics do change properties and some become more brittle (example you drop it and it cracks) but it's not going to just fall apart on you.

One factor that is true with PLA is heat. By that I mean remember you do not have to "melt" plastic, there is a glass transition temperature that is MUCH lower than "melting" temperature but a part loses mechanical strength and can deform. So don't leave in a hot car, don't leave in the sun, don't install an incandescent light bulb or circuit board with a linear regulator that gets hot.

Another thing to keep in mind that a huge change in the last say 7 years, is that hardly anyone uses pure PLA for printing. Yes, back in the day (I started 3D printing circa 2009 when Makerbot was making Cupcake kits) because the industry was beginning we used what industrial sources existed. Now days, there is enough demand and market, PLA used in 3D printing is a blend and extremely specialized to improve the basic properties and suitability. What information you may have gotten was likely about food grade packaging PLA materials- intentionally bragging about how it can biodegrade compared to oil based plastics.

@RoyBoy posted:

Would a coat of good quality paint seal the pores from the air and make the PLA last longer?

Yes, back in the day we used things like XTC 3D, basically a brush on epoxy coating that smoothed and could water tight seal an object. the problem I had with that is it tended to yellow- like many epoxies can do.

For the record, I have PLA based prints that are now easily approaching 10 years old. Still intact, still just fine.

I still have my Grumpy pumpkin in glow in the dark PLA that is the size of most people's 3D printers

Someone (not me) standing in another giant PLA vase I made on my largest 1 meter 3D printer.

Maker Fair

My details on some of my 3D printing exploits https://www.flickr.com/photos/...ms/72157646169607634

I print with PLA, and while I do think it will last for quite some time its still a good idea to prime/paint it.  Like Wood PLA will more readily absorb any humidity in the air which can affect any thin bits over time (recent experience has taught me that).  The longevity of your PLA can also be affected by UV exposure (like any material).