Just drive it in the rain. Its an old car yes but eventually you will probably strip it get it on a rotissorie and do a full ground up resto - not now though.
Until then enjoy the fact that you can get an odd dent in it and its not perfect. Work on the mechanics of it, don't be too precious with it.
I'm speaking from someone who used to stress about my car all the time. The reality is you could be so worried about little things like rust and dents, but unfortunately (hopefully not) some drunk driver or just some idiot drive could come along and smash the whole thing to bits and there you were stressing about a spot of rust.
Yes treat the rust to prevent it getting worse, but drive it in the rain. Its not a show car yet! .
NR240 (Nigels old 240z now owned by Sexual_Sushi) he told me how he used to stress over the car and not drive it in the rain, he said he regretted that and had started driving it in the rain. He has obviously sold it now and probably thinks he was being really silly from the way he was explaining it to me.