Can you refill a helium balloon if it loses some air, without ruining it?

Answer 1

Mylar (the metallic ones)balloons last for weeks, even longer sometimes (and they can be refilled). But the regular balloons will deflate after one day, so you can't blow them up the day before. Some places will add "hi-float" which is a glue-like substance that coats the inside, and with this added, balloons will probably still look OK the next day (not as fresh, but they have a good chance of still being up).

Answer 2

Yes, if it is mylar only. Not rubber....