the 1st thing I would say is does it ultimately matter? If the sign is going to be up in the air or viewed from any distance at all will it even be visible?
obviously you want to exercise as much caution as possible but this is a risk when reusing a substrate (among others) & hopefully your client understands the risks & is willing to accept them for the 'savings' they think they may be receiving..in my experience when you factor in time to remove existing graphics & prepare the existing substrate the savings are minimal at best..there are obviously some circumstances where you have to reuse an existing substrate and that makes the point I just mentioned irrelevant.
work slowly & with caution. Warm up the material with a heat gun,blow dryer,heat lamps/bodyshop lamps,& sometimes even the sun is enough depending on the substrate,the vinyl & how long it has been installed.
materials such as reflective are much more difficult to remove than plain old cast or calendared vinyl.
there are other tools that can be used to remove vinyl but if you are trying to do as little damage to the substrate you are utilizing the proper tools. If you are 'scraping' you will want to change your technique...i find that lifting a corner & lifting, peeing the material to be removed does MUCH less damage than scraping..obviously.
if it is a large piece once you get it started you may choose to 'roll' the vinyl over itself, I have found this to be effective for removing panels that have been 'sheeted' or flooded with vinyl or other large pieces.
if the adhesive comes up with the vinyl,wonderful..bonus..makes the job easier,cleaner & faster but if not simply use the least aggressive adhesive removal as possible & work up to more aggressive removers if necessary but in my experience even the least aggressive removers such as that product that works great but I don't like to endorse (RR) will remove even the most aggressive ld adhesive if you let it sit & give it time to work (obviously test any chemical in a hidden area prior to use so that you do nit create additional problems for yourself...ESPECIALLY if you need to reuse the substrate).
if the damage is substantial & customer will not accept it (which they should be aware of the risks prior to tackling a project of this nature) depending on the substrate you can usually buff out minor scratches, repaint the finish if necessary & appropriate for the type of material.
all this is why I personally have always encouraged clients to replace the material & everyone in the trade that I have trained I stress the same standpoint.
always charge by the hour on projects like this as well, I hate to quote a firm price assuming it will go smoothly & it ends up being a miserable wrestling match removing tiny 1/2"x3/8" slivers of vinyl....no fun.
good luck. Oh & I prefer regular razor blades over lil chizlers (but if you get lazy you can create great damage in the blink of an eye...) just a personal preference.
