You really need to get rid of that olds glass 100% !! the new glass need to be over a sanded surface anyway so grinding the old stuff off is absolutely vital !!. use the coarsest grinding discs you can find !! 24 grit or 36 grit 80 grit is way to fine !! and get rid of all the old loose glass that is there . when you done that post a reply and lets have a look at what you have please !
To get a acceptable bond to the old surface it need scuffing thoroughly all over and really really clean !!! have you thought a about what resin you going to be using with your glass ???