Every time I reuse a jar that I've canned with before, I have to remove
the previous label. The ones I've been using don't wash off with water,
and I guess I'll have to use paint thinner or naptha to remove them. Is
there a label that will just wash off?

I'm finding that the labels they put on jars you get at the market do
wash off, although often I have to boil the jars and use a razor-blade
scraper and then an abrasive scrubber.