Clear the canvas before drawing the gears.

experimental/SkV8Example/gears.js

 var IS_SKV8 = typeof document == "undefined";
 var HAS_PATH = typeof Path != "undefined";
 
 var NumTeeth = 24;
 var NumGears = 60;
 var DeltaTheta = Math.PI/90;
 var FaceColors = ["#000099", "#006600", "#990000", "#EEEE00"];
 var SideColors = ["#0000FF", "#009900", "#FF0000", "#CCCC00"];
 
 function makeGear(pathLike, r) {
@@ skipping 79 matching lines @@
         path: gearPath(r),
         r: r,
         faceColor: FaceColors[color],
         sideColor: SideColors[color]
     });
   }
 
   function draw(ctx) {
     ctx.resetTransform();
 
+    ctx.fillStyle = "#FFFFFF";
+    ctx.fillRect(0, 0, 499, 499);
+
     rotation += DeltaTheta;
     if (rotation >= Math.PI*2) {
       rotation = 0;
     }
 
     for (var i=0; i < gears.length; i++) {
       gear = gears[i];
       draw3DGearAt(ctx, rotation, gear);
     }
 
@@ skipping 19 matching lines @@
     var canvas = document.getElementById("gears");
     var ctx = canvas.getContext("2d");
     function drawCallback() {
       onDraw(ctx);
       setTimeout(drawCallback, 1);
     }
     setTimeout(drawCallback, 1);
   }
 }