The grandfather was suggesting that he, as a professional painter, was superior to and amateur painter because he didn't foolishly imagine that all of his work were masterpieces worth keeping. As a professional, he held himself to a higher standard.
If the (imperfect human) Grandfather/Painter were an imbalanced man, he'd have painted a big red X across the original painting and thrown it in the corner where it was of no use to anybody. He may have even, in a fit of rage, put his fist through some of the canvas that he especially disliked and rendered them forever useless. This kind of behavior would have frightened his family and made them question his sanity. Thankfully the Grandfather/Painter, being a balanced man, realized that he could choose to see the value in the existing painting and simply touch it up to make it more to his liking, or, he could chose to simply start over.
Christians base their belief in the existence of God on the Bible where it tells us he is a supreme ,just and loving being, incapable of error....he is not an amateur. Since we are comparing him to the Grandfather/Professional Painter, if he were truly just and loving God, he could have chosen to "touch up" the first human pair and improve their flaws or he could have simply started over. Instead, he chose to curse them and to render them in even worse condition than before. He tossed them in the corner (so to speak) and left them there to fend for themselves. Later he filled his art studio with water that ruined most of his work. A few of the paintings that he "sort of liked" floated to the top and survived this insane action. However because they weren't entirely perfect, he tossed them in the corner and eventually forgot about them too and vowed that one day, he would be back to burn his entire studio down.