A new chain is mega cheap anyway, just be honest with them about what you have done and be prepared to shell out the cash for a chain or quick link or whatever

EDIT:----
I didn't see the op found the issue .... for some reason his name wasn't highlighted in black and I missed the post.

Well done, sir. Soon you will be enjoying the ride.

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A 45 minute drive .... and another one home, and if you have to leave it, Another ninety minutes .... but still.

have you ever heard of someone fixing a chain with sandpaper? You know why not? Because there is no way to fix a chain with sandpaper. You can cover up some damage, but you haven't fixed anything .... whatever is causing the damage is still there. it's like putting new tubes in a bike over and over .... put in the tube after a flat, pump it up, it leaks, repeat, repeat, repeat ...... at some point take the piece of glass out of the tire.

I have no idea what is happening with your chain. But you don't either. Sure you could drop $20 on a chain each week .... or take the time to take the bike back to the shop and make Them fix it.

If they sold you a defective bike and you Want a defective bike .... cool. Sorry I interuded.

if you want a bike you can ride ... and you assumed that is what they sold you ... Take The Bike Back to the Shop. Make them fix it for free.

or ... stock up on chains.