Aloha Johan, Jamie and others
If the canvas backing tears up it is probably because you are not removing the old piece of sandpaper while it is still hot (temperature wise) from use. The glue will be softer and willing to release. If you sand in a way that doesn’t get the glue hot but does dull the paper, you can still heat it up by setting up a sacrificial piece of plywood on your work counter. Hold the pad flat on it and press hard as up ease up to high RPMs to quickly heat up the sandpaper and glue. Don’t be afraid to get it HOT, when the wood smokes, the sandpaper will come off real easy. Then modify your technique/system to remove your old sandpaper as soon as you stop sanding on the board and it is still hot.
Jamie, Super 77 is WAY too powerful to use for sticking on sandpaper. Use the specialized adhesives made for the task. In the old days it was Glop, now it is Taki and 3M Feathering Disk Adhesive. There may be others but these are the common ones that jump to mind at the moment.
If per chance, you get a buildup of old paper and glue on a pad from time to time (and you will for sure with Super 77) turn it face up and use a piece of wood rounded on the end a bit. Press this firmly near the outer edge of the pad while spinning it at about 3000rpm or more. Move it around a bit to CAREFULLY heat the glue in that area. You will see the glue soften and you can then work this heat wave in toward the center of the pad gathering the old glue and paper with it as you go. It will ball up and fly off, just keep going into the center of the pad where you can grab off the final glob. Just don’t press too hard or wear into the canvas cover and rip it.
If you get a tear in the canvas or at the edge of the pad from catching it on a fin or something, you can glue it back in place with contact cement. But the pad will never be the same so always calculate for some loss or damage to the pads and replace them from time to time. Also, the pad’s foam hardens over time and it will also loose some of it’s consistency such that the fastest and most accurate sanding will be done with pads that are newer.
Always cut your paper in advance, leaving a minimum 3/8" overhang over the edge of the pad. Rarely will pre cut sheets have enough overhang. And they typically won’t have the quality of abrasive on them that you can get in sheets for much cheaper. I prefer Hermes.
Another tip is to use a piece of 60 grit sandpaper rolled up and carefully sand a 1/2" arc into the side of the sanding pad’s foam, just below the canvas edge, leave some foam tapering out under the canvas side. This and the sandpaper’s overhang will give you a consistent release across the pads edge that will keep you from making “swirlies” or gouges in your work. And if you are sanding glass on fins it will allow you to 90% sand them out with the machine as the pads edge will conform better to the fillet at the base of the fin.
Often sanding pads or grinding discs aren’t truly round or true to center. Use a sanding block with 60grit and while spinning the new pad press it up against the pads edge and angular side to true up the pad. This will remove a lot of uncomfortable vibration and allow you to work longer and faster with less fatigue. Plus it will allow you to be very accurate with the edge of the pad when doing detail work in channels, wings or glass on fins, etc. All pads will need this from time to time also, as they age.
Hope this helps