Index: zorg/buildbot/builders/ClangBuilder.py
===================================================================
--- zorg/buildbot/builders/ClangBuilder.py
+++ zorg/buildbot/builders/ClangBuilder.py
@@ -609,6 +609,11 @@
     stage2_build = 'stage2'
     stage2_install = 'stage2.install'
 
+    # The installation step can be skipped if we aren't doing a multi-stage
+    # build, an upload, or an LNT test.
+    should_install_stage1 = (useTwoStage or runTestSuite or
+                             stage1_upload_directory)
+
     # Set up VS environment, if appropriate.
     if vs:
         f.addStep(SetProperty(
@@ -617,17 +622,35 @@
         assert not env, "Can't have custom builder env vars with VS"
         env = Property('slave_env')
 
+    # Allow users to force clean builds from the builder page by setting the
+    # "clean" property.
+    clean_build_requested = lambda step: step.build.getProperty("clean") or clean
+
 
     ############# CLEANING
-    if clean:
-        f.addStep(ShellCommand(name='clean stage 1',
-                               command=['rm','-rf',stage1_build],
+    f.addStep(ShellCommand(name='clean stage 1',
+                           command=['rm','-rf',stage1_build],
+                           warnOnFailure=True,
+                           description='cleaning stage 1',
+                           descriptionDone='clean',
+                           workdir='.',
+                           doStepIf=clean_build_requested))
+
+    if should_install_stage1:
+        # Clean the installation directory as well to remove stale build
+        # artifacts that no longer exist, such as old clang resource
+        # directories.
+        f.addStep(ShellCommand(name='clean stage 1 install',
+                               command=['rm','-rf',stage1_install],
                                warnOnFailure=True,
                                description='cleaning stage 1',
                                descriptionDone='clean',
                                workdir='.',
-                               env=env))
-    else:
+                               doStepIf=clean_build_requested))
+
+    if not clean:
+        # On builders that are normally incremental, test if build.ninja already
+        # exists to see if we can avoid an extra cmake run.
         f.addStep(SetProperty(name="check ninja files 1",
                               workdir=stage1_build,
                               command=check_build_cmd,
@@ -666,7 +689,7 @@
                                    workdir=stage1_build,
                                    env=env))
 
-    if useTwoStage or runTestSuite or stage1_upload_directory:
+    if should_install_stage1:
         f.addStep(ShellCommand(name='install stage 1',
                                command=ninja_install_cmd,
                                description='ninja install',
@@ -689,17 +712,24 @@
 
     ############# STAGE 2
     if useTwoStage:
-        # We always cleanly build the stage 2. If the compiler has been
-        # changed on the stage 1, we cannot trust any of the intermediate file
-        # from the old compiler. And if the stage 1 compiler is the same, we
-        # should not build in the first place.
+        # We always cleanly build the stage 2. If the compiler has been changed
+        # on the stage 1, we cannot trust any of the intermediate files from the
+        # old compiler. And if the stage 1 compiler is the same, we should not
+        # build in the first place.
         f.addStep(ShellCommand(name='clean stage 2',
                                command=['rm','-rf',stage2_build],
                                warnOnFailure=True,
                                description='cleaning stage 2',
                                descriptionDone='clean',
-                               workdir='.',
-                               env=env))
+                               workdir='.'))
+
+        f.addStep(ShellCommand(name='clean stage 2 install',
+                               command=['rm','-rf',stage2_install],
+                               warnOnFailure=True,
+                               description='cleaning stage 2',
+                               descriptionDone='clean',
+                               workdir='.'))
+
 
         # Set the compiler using the CC and CXX environment variables to work around
         # backslash string escaping bugs somewhere between buildbot and cmake. The
@@ -719,7 +749,6 @@
                                haltOnFailure=True,
                                description='cmake stage 2',
                                workdir=stage2_build,
-                               doStepIf=lambda step: step.build.getProperty("exists_ninja_2") != "OK",
                                env=env))
 
         f.addStep(WarningCountingShellCommand(name='build stage 2',