diff --git a/src/org/nschmidt/csg/CSG.java b/src/org/nschmidt/csg/CSG.java
index d5b45110..51a2c791 100644
--- a/src/org/nschmidt/csg/CSG.java
+++ b/src/org/nschmidt/csg/CSG.java
@@ -215,15 +215,18 @@ public CSG union(CSG csg) {
         });
 
         CompletableFuture<Node> f1 = CompletableFuture.supplyAsync(() -> new Node(thisPolys));
-        CompletableFuture<Node> f2 = CompletableFuture.supplyAsync(() -> new Node(otherPolys));
-        CompletableFuture.allOf(f1, f2).join();
+        CompletableFuture<Node> f2 = CompletableFuture.supplyAsync(() -> new Node(this.createClone().polygons));
+        CompletableFuture<Node> f3 = CompletableFuture.supplyAsync(() -> new Node(otherPolys));
+        CompletableFuture.allOf(f1, f2, f3).join();
 
-        final Node a;
+        final Node a1;
+        final Node a2;
         final Node b;
 
         try {
-            a = f1.get();
-            b = f2.get();
+            a1 = f1.get();
+            a2 = f2.get();
+            b = f3.get();
         } catch (ExecutionException e) {
             // Exceptions should (tm) already be thrown by the "join()" call.
             throw new LDPartEditorException(e);
@@ -232,15 +235,12 @@ public CSG union(CSG csg) {
             throw new LDPartEditorException(e);
         }
 
-        a.clipTo(b);
-        b.clipTo(a);
-        b.invert();
-        b.clipTo(a);
-        b.invert();
+        a1.clipTo(b);
+        b.clipTo(a2);
 
         final List<Node> nodes = new ArrayList<>();
         final Deque<NodePolygon> st = new LinkedList<>();
-        st.push(new NodePolygon(a, b.allPolygons(new ArrayList<>())));
+        st.push(new NodePolygon(a1, b.allPolygons(new ArrayList<>())));
         while (!st.isEmpty()) {
             NodePolygon np = st.pop();
             Node n = np.node();
@@ -251,7 +251,7 @@ public CSG union(CSG csg) {
             }
         }
 
-        final List<Polygon> resultPolys = a.allPolygons(nonIntersectingPolys);
+        final List<Polygon> resultPolys = a1.allPolygons(nonIntersectingPolys);
         return CSG.fromPolygons(resultPolys);
     }
 
diff --git a/src/org/nschmidt/csg/Node.java b/src/org/nschmidt/csg/Node.java
index 1e21e8e7..eefb1184 100644
--- a/src/org/nschmidt/csg/Node.java
+++ b/src/org/nschmidt/csg/Node.java
@@ -91,6 +91,10 @@ public Node(List<Polygon> polygons) {
                     st.push(np2);
                 }
             }
+
+            if (it == 10000) {
+                NLogger.debug(getClass(), "CSG limit exceeded."); //$NON-NLS-1$
+            }
         }
     }
 
diff --git a/src/org/nschmidt/csg/Plane.java b/src/org/nschmidt/csg/Plane.java
index 42e7dba8..39e9e5ef 100644
--- a/src/org/nschmidt/csg/Plane.java
+++ b/src/org/nschmidt/csg/Plane.java
@@ -114,12 +114,29 @@ int[] getTypes(final Polygon polygon) {
         final int size = polygon.vertices.size();
         final int[] types = new int[size + 1];
         int polygonType = 0;
+        int coplanarityHits = 0;
+        boolean noCoplanar = false;
         for (int i = 0; i < size; i++) {
             double t = this.normal.dot(polygon.vertices.get(i)) - this.dist;
             int type = t < -Plane.epsilon ? BACK : t > Plane.epsilon ? FRONT : COPLANAR;
+            if (type == COPLANAR) {
+                coplanarityHits += 1;
+                if (coplanarityHits > 2 && noCoplanar) break;
+            } else {
+                noCoplanar = true;
+            }
             polygonType |= type;
             types[i] = type;
         }
+
+        // When three or more points are co-planar, then the whole polygon is co-planar!
+        if (coplanarityHits > 2 && noCoplanar) {
+            polygonType = COPLANAR;
+            for (int i = 0; i < size; i++) {
+                types[i] = COPLANAR;
+            }
+        }
+
         types[size] = polygonType;
         return types;
     }