So for quite sometime, between now and April of 2020... all... of my HPF Trigger Start ballasts, and most of my PL-L 18w HPF Rapid Start ballasts, have been sitting in boxes in my garage, next to my sportcar since 2021. At this point, since they've been sitting outside for so long, a part of me is afraid to test them out, especially the 10 Trigger Start ballasts that I've NEVER used, since I bought them back in like 2018. All of my GE Bonus Line Trigger Start ballasts have not seen use since 2020, when I moved into this apartment complex. Should all of these ballasts still be okay? Or is it risky for me to use them again at this point? Probably would've been a good idea for me to bring all of those ballasts into my apartment/find space for those boxes inside.
**EDIT, I should add that all boxes have been dry -- they've never seen wet conditions. Hot and humid conditions though, and extremely cold temperatures, have unfortunately been evident.

I would think they would be fine. Even if they were to get a little wet I would probably still try using them.

@Multisubject, this is what I'm thinking as well. There's a small additional lighting project that I'm thinking about doing with LED light tables,
that would involve two of my rare cold temp HPF magnetic Trigger Start ballasts for smaller T12 or T8 lamps. Unfortunately though, one of those ballasts
has a slight bit of rust development. I plan on bringing in all five of those GE Bonus Line ballasts to see if they all work, but I know that negative
results can prove to be catastrophic.

Here is how I like to think about it. You have two options in my mind:

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1):
Keep them because they are rare, and never put them to use.

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2):
Use them because they are useful. If they end up failing, it doesn't really matter because you can still keep them for their rarity. If you were gonna keep them boxed up because they are rare, then it doesn't matter if they are working or not because they aren't being used.

Now of course if they do fail while in use, there is a possibility that it will leave the ballasts visually disfigured, which will make them not-so-ideal for keeping around as rarities. But if they are thermally protected you should be fine. Maybe put a fuse in line to protect them if you feel the need, fuses and fuseholders are very cheap.

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I know my stance is probably a little more utilitarian than other users here, but to me it makes sense. Obviously I am a stranger and you don't have to listen to me, but the stakes here are low in my opinion.

Not to mention that these ballasts are almost always potted in tar, and "a slight bit of rust development" is almost certainly nothing to worry about in the first place. Even if it was completely covered in rust, I would still try it out (with proper safety precautions of course). Do you have a picture of the rustiest one of them all?

No recent pics of those ballasts yet, as I've simply been in and out of that garage with the Genesis, but I will be taking pics of all five of those GE Bonus
Line ballasts in their current state, soon. I do have pics of those ballasts in the past though, somewhere in my gallery.
4 of them are 0.32 amp cold temp Trigger Start, and one of them is a regular 0.28 Trigger Start. All of them lit both T8 and T12 lamps very bright...
they are some of my best ballasts in my collection... so if I do test them and the tests all pass, I'll definitely find a use for all of them at some point.
The other 10 that have never been used, are all Valmont Bonus Line 0.28 amp Trigger Start ballasts.