diff --git a/build.py b/build.py
index 3761ccc9a..cae794207 100755
--- a/build.py
+++ b/build.py
@@ -1144,6 +1144,17 @@ def cmd_etg(options, args):
         etgfiles.remove(core_file)
         etgfiles.insert(0, core_file)
 
+    # We need two loops:
+    # 1. Cache all type auto conversion rules which get created in etgtools/tweaker_tools.FixWxPrefix
+    # 2. Actually create the sip files
+    # This is needed because each etg file is run in its own python invocation, so
+    # the data is lost between them.
+
+    # First delete the old cache if found
+    cacheFile = opj(cfg.ROOT_DIR, 'sip', 'gen', '__auto_conversion_cache__.json')
+    if os.path.isfile(cacheFile):
+        os.remove(cacheFile)
+    is_newer = {}
     for script in etgfiles:
         sipfile = etg2sip(script)
         deps = [script]
@@ -1157,7 +1168,16 @@ def cmd_etg(options, args):
 
         # run the script only if any dependencies are newer
         if newer_group(deps, sipfile):
+            is_newer[script] = True
+            runcmd('"%s" %s %s' % (PYTHON, script, flags + " --only-cache-auto-conversions"))
+        else:
+            is_newer[script] = False
+    for script in etgfiles:
+        if is_newer[script]:
             runcmd('"%s" %s %s' % (PYTHON, script, flags))
+    # Delete the cache
+    if os.path.isfile(cacheFile):
+        os.remove(cacheFile)
 
 
 def cmd_sphinx(options, args):
diff --git a/etgtools/pi_generator.py b/etgtools/pi_generator.py
index 840115ea6..c442d6a87 100644
--- a/etgtools/pi_generator.py
+++ b/etgtools/pi_generator.py
@@ -571,19 +571,45 @@ def generatePyProperty(self, klass, prop, stream, indent):
     def _generateProperty(self, klass: extractors.ClassDef, prop: Union[extractors.PyPropertyDef, extractors.PropertyDef], stream, indent: str):
         if prop.ignored or piIgnored(prop):
             return
+        # Track separate value and return types.
+        # This prevents us from emitting Union types on the getter, which do not make sense in practice,
+        # as exactly one type will be returned, even if multiple types are allowed by the setter.
         value_type = ''
+        return_type = ''
         if prop.getter:
             getter = self.find_method(klass, prop.getter)
             if getter and getter.signature:
-                value_type = getter.signature.return_type
+                return_type = getter.signature.return_type
         if prop.setter:
             setter = self.find_method(klass, prop.setter)
-            if setter and setter.signature:
-                value_type = setter.signature[0].type_hint
+            # The setter can be overloaded and we should find all setters which have exactly one argument and Union their types.
+            # We need to do this, because the property-setter only has one argument, so we assume that all overloads which
+            # take more than one argument are not applicable here.
+            if setter:
+                value_types =[]
+                if setter.signature and len(setter.signature._parameters) == 1:
+                    value_types.append(setter.signature[0].type_hint)
+                if setter.hasOverloads():
+                    for overload in setter.overloads:
+                        if not overload.ignored and overload.signature and len(overload.signature._parameters) == 1:
+                            value_types.append(overload.signature[0].type_hint)
+                if len(value_types) == 1:
+                    value_type = value_types[0]
+                elif value_types:
+                    # This does not flatten already existing Unions, but this is allowed
+                    value_type = f'Union[{", ".join(value_types)}]'
+        # We choose some default values if no types were found above. This may lead to wrong info, if the underlying getter and setter types are wrong to begin with
+        if value_type or return_type:
+            if not value_type:
+                value_type = return_type
+            elif not return_type:
+                # We probably should not choose the value_type as default if it contains a Union,
+                # but this might still be better than using Any.
+                return_type = value_type
         if prop.setter and prop.getter:
-            if value_type:
+            if value_type and return_type:
                 stream.write(f'{indent}@property\n')
-                stream.write(f'{indent}def {prop.name}(self) -> {value_type}: ...\n')
+                stream.write(f'{indent}def {prop.name}(self) -> {return_type}: ...\n')
                 stream.write(f'{indent}@{prop.name}.setter\n')
                 stream.write(f'{indent}def {prop.name}(self, value: {value_type}, /) -> None: ...\n')
             else:
@@ -591,7 +617,7 @@ def _generateProperty(self, klass: extractors.ClassDef, prop: Union[extractors.P
         elif prop.getter:
             if value_type:
                 stream.write(f'{indent}@property\n')
-                stream.write(f'{indent}def {prop.name}(self) -> {value_type}: ...\n')
+                stream.write(f'{indent}def {prop.name}(self) -> {return_type}: ...\n')
             else:
                 stream.write(f'{indent}{prop.name} = property({prop.getter})\n')
         elif prop.setter:
diff --git a/etgtools/tweaker_tools.py b/etgtools/tweaker_tools.py
index 58ca5f3af..633c82e63 100644
--- a/etgtools/tweaker_tools.py
+++ b/etgtools/tweaker_tools.py
@@ -20,7 +20,7 @@
 import sys, os
 import copy
 import textwrap
-from typing import NamedTuple, Optional, Tuple, Union
+from typing import Final, NamedTuple, Optional, Tuple, Union
 
 
 isWindows = sys.platform.startswith('win')
@@ -246,6 +246,24 @@ def removeWxPrefix(name):
     return name
 
 
+# Filename for type auto conversion cache file 
+_AUTO_CONVERSION_CACHE_FILE: Final = '__auto_conversion_cache__.json'
+
+
+def load_auto_conversions(destFile: Optional[str] = None) -> dict[str, Tuple[str, ...]]:
+    """Load FixWxPrefix auto conversions from cache file if it exists."""
+    import json
+    if not destFile:
+        from buildtools.config import Config
+
+        cfg = Config(noWxConfig=True)
+        destFile = os.path.join(cfg.ROOT_DIR, 'sip', 'gen', _AUTO_CONVERSION_CACHE_FILE)
+    # Need to merge existing data with current data
+    if os.path.isfile(destFile):
+        with open(destFile, 'r', encoding='utf-8') as f:
+            return json.load(f)
+    return {}
+
 
 class FixWxPrefix(object):
     """
@@ -254,7 +272,22 @@ class FixWxPrefix(object):
     """
 
     _coreTopLevelNames = None
-    _auto_conversions: dict[str, Tuple[str, ...]] = {}
+    _auto_conversions: dict[str, Tuple[str, ...]] = load_auto_conversions()
+
+    @classmethod
+    def cache_auto_conversions(cls, destFile: Optional[str] = None) -> None:
+        """Save current auto conversions to a cache file."""
+        import json
+
+        if not destFile:
+            from buildtools.config import Config
+
+            cfg = Config(noWxConfig=True)
+            destFile = os.path.join(cfg.ROOT_DIR, 'sip', 'gen', _AUTO_CONVERSION_CACHE_FILE)
+        # We overwrite the cache file, as we should have loaded the existing values when
+        # initializing the class.
+        with textfile_open(destFile, 'wt') as f:
+            json.dump(FixWxPrefix._auto_conversions, f)
 
     @classmethod
     def register_autoconversion(cls, class_name: str, convertables: Tuple[str, ...]) -> None:
@@ -371,6 +404,7 @@ def cleanType(self, type_name: str, is_input: bool = False) -> str:
             'time_t': 'int',
             'size_t': 'int',
             'Int32': 'int',
+            'Uint32': 'int',
             'long': long_type,
             'unsignedlong': long_type,
             'ulong': long_type,
@@ -950,6 +984,9 @@ def runGenerators(module):
     checkForUnitTestModule(module)
 
     generators = list()
+    if '--only-cache-auto-conversions' in sys.argv:
+        FixWxPrefix.cache_auto_conversions()
+        return
 
     # Create the code generator selected from command line args
     generators.append(getWrapperGenerator())