I must admit I've not had the problem Howzatt but I have a suspicion the only reliable way forward will be to fill the hole with a fine grade body filler (Milliput perhaps) and re-drill and re-tap the thread. Obviously if the screw is of the self tapping type it won't be necessary to re-tap the hole but care will be necessary to drill the correct size hole. By this I mean small enough for the screw to form a thread capable of holding the screw in place but not so small as the screw breaks out the filler and splits the post.