I think now that you have sanded ( I wouldn't have done this ), you should soak the knife in a bath of Mineral oil, before we go too much into this - Charlie was pretty spot on with his advice as well, if its a rivet problem-leave it to the experts.
Anyway.. soak the knife in Mineral oil-throw your sandpaper away-you are going to ruin your knife!!!
Leave the knife to soak for a week, take it out, and work both blades like crazy with the oil getting in everywhere.....
re-soak the knife for a day and night-again take it out and work those blades back and forth..
Next step, used warm to near hot tap water and dishwashing liquid....squirt d/washing liquid into joints...again work those blades like crazy, then hold the knife joints under the soft running tap, and wash that d/washing liquid out...repeat this exercise using the d/washing liquid.
After this take your knife out to your garage and use compressed air and air gun to blow out all the water out of the joints of your knife....
Grab a can of WD40 ( because this repels water )and spray liberally into the joints, and work them back and forth like crazy.....
Use your air gun again to get out all the WD40 you can, go grab your Mineral oil ( Baby oil ) and a few drops into each joint...again work this into the joint- you will soon see the oil coming out along the back springs - this should let you know your oils in well and truly.
Now this sounds like a real hassle to do, but if you are in any way suspect about a joint-if this process doesn't make it better ( because it has done to most of my seized knives - a huge percentage of them) then your joint is questionable, you then would go down the track of which Charlie has said.
Again, don't use your sandpaper - if you follow this method, you are only cleaning, done properly you can only better your knife, using sandpaper - you will only ruin your knife.
Good luck.