May I suggest a different strategy? Tape cardboard to the door temporarily and then never, ever respond to your dogs' scratchings no matter how long they persist. Once they (eventually) realize their behavior isn't ever rewarded by the opening of the door, they will learn not to scratch. If you want to train them to use a different signal, you could put a bell on a string for them to ring and teach them that the door opens when they do that, rather than scratching. But I suspect you would prefer that they not beg to come in at all, in which case non-response is the best solution. It will take awhile, however, to break their habit. If you want to speed up the process, try making a loud noise they don't like every time they scratch.