Using nail polish to touch-up scratched/rusted frames is more or less well-known (cf. Girls Biking to Work), but recently a hobbyist restorer of old bikes told me that you can "just apply clear coat over the entire frame" in order to keep the frame from rusting and to protect the paint job. Is this true?— What are the implications of applying a layer of clear coat over an old paint job on an old frame? I would be afraid that e.g. the clear coat would somehow discolor the paint job or perhaps cause the old paint job to somehow come off or something like that.
You can but there are a couple things you will have to do first. Like @FreeMan was saying, to do it properly you would need to thoroughly clean and I would also use a degreaser before you paint it.
Something you could try before using the clear coat would and adhesion promoter like this. Even after doing all of that, there is no telling how well it will work and/or how long it will last.
Without a proper paint booth or ideal conditions it can be really easy to screw up a paint job.
If there is any underlying rust that hasn't presented itself, it will eventually even with a new coat.
Your best bet if you really want to do a proper job, is to sand, prime, paint again, and then use a clear coat.
If you want preserve the paint then you may be out of luck. If you want to simply restore/reuse the frame then I would just go back and do a proper job. It all depends on if it's worth the time, money, and energy to you.
You could, but I wouldn't recommend it if the ultimate goal is preservation:
If you just clear-coat using a spray can from the hardware store, you're unlikely to have good (as in both visual and long-lasting) results. After a while, the consumer clear coat will yellow and/or flake off. However, you can't beat the price of under $5 / 5 €.
You could get it professionally done at a body-shop, but that will be expensive.
The main problem with clear-coat is that it takes away all of your future options. You can't paint over the clear coat so you won't be able to paint the bike again without taking off the clear coat, which means taking off all of the paint, taking the bike back down to bare metal and repainting.
If you're still vacillating and/or saving up money about what to do, I would do minor touchups using touch-up paint (available at an auto hardware shop) and instead cover the major parts of the frame with helicopter tape (also called Racer's tape). It's relatively cheap, durable, and most importantly -- easily removable.