Getting Rid of Chemicals.

I don't like using chemicals in the garden but have finally bought some Roundup for the weeds on my patio which have got the better of my arthritic hands and back.
My husband mixed it up last week but then used my watering can to apply it. (I intended to fill an empty spray container which had window cleaner in it.)
How do I clean out my watering can? I have rinsed several times with clean water but am not confident its safe to use on my seedlings now.