Hold on there for a minute. Do you mean you were laying up glass and then had to quit for a while and now you're getting back on the job, or do you mean you have an old glass part that you wish to now modify by doing additional glass work? It also matters if the resin that was originally used was epoxy or polyester. Different questions qet different answers.
If its the part was made using epoxy resin, and you've been working on a piece but set it aside for a while and now are resuming glass-work on it here's what you have to do. Wash it with hot water and dish soap and a good stiff brush. Let it dry, then do the acetone wipe down and proceed. If its a polyester piece you can forgo the soap and water wash but you will need to roughen the surface with sandpaper in its place.
Here are the reasons. An epoxy resin will release a chemical for about a day after is been layed up that migrates to the surface, its called an amine blush. That upper surface will inhibit adhesion of subsequent layers. So if you stop for a day you have to get it off - which can be done with a soap and water scrub. When the blush is gone epoxy resin will bond very very well to itself. On the polyester pieces (which is the most common resin used because its way way cheaper than epoxy) the stuff is not particularly good at bonding to itself (putting new on old) and you need to give the old piece some "tooth" to get a good mechanical bond. So you have to sand it (80p grit) and then clean it before going forward.
If its an old glass part, lets say a used tank you picked up on E-Bay, then assume its made with polyester resin, sand the area you wish to modify with 80 grit in prep, wipe down with acetone as most everyone has recommended and then have at it.
