Dwight News wrote:
I am refinishing a canoe paddle and would like to use oxalic acid on the
dark spots.

1. Can I still use the acid I have had for 25 years?

Shouldn't be a problem.

2. What proportion should I mix with water?

Usually you just make a saturated solution. Start with hot water, and
dissolve as many crystals as you can (stirring until your arms are sore helps).
As the water cools, you'll get a bit of precipitate...no big deal.