IMHO, this is the primary reason why many people have trouble patching tubes and many don't understand that this is the primary reason for sanding. You MUST get that compound off or your patch job WILL fail. I always aggressively sand the tube all the way to, and beyond, where the patch edge will be using sandpaper, not those worthless metal scratch thingies that come in some patch kits. The second reason that patches fail is that folks don't wait long enough until the vulcanizing fluid skins over. I have noticed zero difference in recent vs prior patch job experiences. They all work perfectly as long as you do it properly.