Before I knew better, I spooled up some spinning reels with mono but I didn't use enough to fill up the spool properly. Can I tie on more mono using a uni to uni, or should I pull it all off and start over with new?

Thanks!

Yes. Depending on how much line you have on the spool, I would remove much of the line so that the knot doesn’t come into play on a cast. I’d remove enough to have the spool only 10-20% left with the new line on top. This would also be the spot I will tie on new line when the time comes. 

 

If you want to get the most out of your spool, make sure it is optimally filled. ?

OR.....depending on how much line you have on the spool, you can take the existing line off the spool by winding it on to a paper towel tube or TP tube, then put some old line on the spool as filler and reload your mono.  If you don't have close to a full spool of mono on there now, it would be easier just to strip some of it off down to the amount of backing you need and re-spool some new mono over it to get to a full spool.