I'm so sad this happened! I'd be so frustrated if that happened to me!
There are four commonly used plastics in 3D Printing: PLA, PLA+, PETG, and ABS.
PLA has a very low melting point and glass transition temperature, almost no shrinkage, and no weird chemical reactions most of the time. It's also a very hard and rigid plastic. This means it's very easy to 3D print because the printer doesn't have to get as hot, you don't have to worry about it chemically bonding to the print bed, and you don't have to worry about the middle of the print cooling off and warping before you're done. Your prints will be as dimensionally accurate as your printer can manage. It's also very durable for a single-piece part. If I wanted a wedge to prop a door open, for example, PLA would probably last longer than the other options.
It has downsides, though. First, it will start to deform around 60 degrees C. Now, if you'd just left a bare bit of plastic outside you probably would have been fine but paint - especially dark coloured paint - will absorb heat. Almost certainly it's that combination of direct sunlight and dark paint that caused your specific issue. PLA is also very difficult to sand, as I'm sure you've discovered, because it's such a hard plastic. This means it's a lot more work to finish for painting. Finally, PLA doesn't solvent-weld well so joining parts involves gluing, which is inherently weaker and and complicates painting.
PLA+ is PLA with something added to it to try and mitigate one or more of PLA's downsides. The most common is the low deformation temperature; the next most common is the inability to solvent weld. But PLA+ is not a standard, so every manufacturer's PLA+ is different and it is completely impossible to make generalisations about PLA+.
PETG's melting point is higher than PLA's but still low enough that most printers can handle it, and there's no real risk of a PTFE lined hotend (which is very common, especially in Creality printers) turning into nerve gas. It is also low-shrinkage and thus low-warp, so it's almsot as easy to print as PLA. It has a much higher deformation temperature, up around 90 degrees C. It's not quite as rigid as PLA which makes it easier to sand, and it's just a tiny bit flexible which means it's a better choice for something that will be screwed or bolted together - the plastic flexes around the bolt rather than being chewed up over time.
PETG can be solvent-welded but this involved MEK, which is incredibly toxic and a controlled substance in many jurisdictions. PETG is also a little harder to paint, since many paints don't stick well to PET and you're more likely to find a spray paint that just never cures on PETG. (Not very likely, mind you, but more likely than with PLA.) Finally, PETG has an unfortunate tendency to fuse with both glass and PEI, requiring some extra work to keep it from becoming permanently attached to the print bed.
ABS has a much higher melting temperature, meaning that there is a real risk of a PTFE lined hotend becoming a neurotoxin. ABS is thus usually printed with all-metal hotends. ABS also shrinks quite a bit when it cools, which means you need to oversize things slightly to get them to be mechanically accurate, and also means that the middle of a print will shrink when the top and bottom don't, so you get all kinds of warping and splitting issues if the print bed isn't enclosed (and often heated). Also, ABS has much more aggressive fumes when heated, requiring a ventilated space. (In reality, all filaments release microplastics and toxic fumes, and you really should not spend all day next to a running 3d printer, but the effects of PLA and PETG are much less noticable. We probably won't know for 30 years which is actually the most dangerous.)
On the upside, ABS is easy to solvent-weld, which allows for very strong joins without foreign materials involved - which in turn makes it that much easier to paint. It's also relatively soft, making it much easier to sand smooth (although this also makes it a bad choice for inserting screws into.) And it's extremely temperature tolerant, with a deformation temp over 100 degrees C. In short, ABS is probably the most difficult of the main three types to actually print, but by far the easiest to join, finish, and paint if you're making something that needs to hide its 3d printed nature.
All of this is, I'm sure, cold comfort, but it may be of use to the next person. And in six months when you're ready to return to this project, perhaps this information will help.
