Scraping the paint off your canvas doesn't always mean you've goofed up. Scraping is a good tool to use between stages of a painting. When you're outside and are quickly blocking in a painting with thick paint because thicker paint gives you a better suggestion of light than thinner paint and makes a bolder statement, you can use a palette knife to scrape an area that you want to refine and it's easier to refine over thinner paint. Scraping dosen't ruin the color, value, or shape, it just gets rid of excess paint. It also teaches you not to fall in love with an area too soon. In the studio I scrape between the block in stage and the broken color stage or if I have to stop in the broken color stage or if I have to stop painting for the day and don't want the painting to dry with thick paint yet, I can go in with a palette knife and lightly scrape excess paint off and not lose the drawing, value or color.