I have a used 120 in the junk room. I was cleaning and stripping old silicone simultaneously, when the cleaning made me aware of scratches I had no clue were there when I purchased the tank. I had to ask myself if the tank was worth the re-sealing effort. Stopped doing the work, stuck the tank in the junk room and it has been sitting there for a couple years. I already had bought a few things of black silicone, so I've just been using it for other stuff like plumbing, making my external overflow box, and repairing air beds.
If you are going to put new silicone, you have to get every tiny trace of the old surface stuff off without cutting into or damaging the stuff holding the panes together. You need a few very fresh sharp razor blades and rubbing alcohol. If you are just worried about aesthetics, you might want to just trim loose stuff so algae doesn't grow under it and then let coralline grow over the stuff (I happen to like a lot of the pink, but have always had so many asterinas that they kept the tanks free of it).