It depends on what you want by 'manipulating'. I assume you want to color and further render the car. You should not do that in the same layer as the sketch is on, just make a new layer and set it to 'multiply' mode. Layers are free, use lots of them (and name them). If you command-click on the mask (or select the mask and choose 'make selection', do not put in a feathering value) you will get a selection where the white parts are 100% selected and the black parts not. If you want to paint one part of the car, you make a new layer. If you already made a mask (by clicking on the circle in the square button), you will use your selection to fill it, or paint within the selcection. If you first make the selection and then click the mask button, the mask is instantly created with the selection you made. You can also cut and paste (but there is no real good reason). Go to the alpha channel, select all and copy. Go to the layer where you want a mask (NOT the layer with the sketch, keep that seperate), make a mask, alt-click you 'jump' inside the mask, so your canvas goes white, and paste the alpha channel. Good luck!