Anyone with a passing familiarity with the art world has probably heard of someone finding a painting by a famous artist under that of some relative unknown. Occasionally people have paid a bargain price for a painting that later turned out to be valuable because of a painting underneath. I even heard once of a painting by a well-known artist that had a painting of an equally well known but older artist underneath (which led to the question, which should we keep?). Such a painting, with another one underneath, is known as a pentimento.
There is a spiritual application to the concept, as well. For better or worse, we paint a picture with our lives. For some it may be like the Picture of Dorian Grey, which showed the debauchery of its model even though his body never showed his sins. For others it may not be as bad. Nevertheless, it is a picture marred here and there with the jarring brushstrokes of sin. No matter how well we try to paint our picture, sin grabs our hand and paints a broad stroke of color across the scene we have painted. "Therefore, as by one man sin entered into the world, and death by sin; so death passed upon all men, for all have sinned." (Rom 5:12) Sometimes nobody else sees what the brush has done; at other times it is known to everyone, except maybe ourselves.
This is where the concept of pentimento comes in. We are, at best, amateur painters. At worst we may be botching a "paint-by-number" canvas. God, on the other hand, sees only a canvas that he can paint on. He sees our poor efforts at painting, but He has a grander art in mind. He is willing to paint over our poor efforts and create the most valuable pentimento of all time.
Some painters use watercolors; others use caseins or oils. There are some artists who make their own paints from natural materials such as soil and plant pigments. God uses a natural paint, as well. He uses blood. Not just any blood, though. God paints over our sins with the blood of His own son. "But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanses us from all sin." (1 Jn 1:7) "Unto him that loved us, and washed us from our sins in his own blood, and hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever." (Rev 1:6) The really strange thing is that instead of getting red, God's blood-based paint comes out white. "Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool." (Isa 1:18)
My mother, the painter, taught me to prepare a canvas by laying down a base coat of white, then add the color. This is what God does for us. Over our sins he lays a base coat of white. Then we can try again to paint our picture. And if we mess up, he lays down another coat of white for us to paint over. When we mess up, God says the process is simple: "Repent, repaint."