diff --git a/3d-viewer/3d_canvas/create_layer_items.cpp b/3d-viewer/3d_canvas/create_layer_items.cpp
index db1b654322..744a2af9b7 100644
--- a/3d-viewer/3d_canvas/create_layer_items.cpp
+++ b/3d-viewer/3d_canvas/create_layer_items.cpp
@@ -686,19 +686,24 @@ void BOARD_ADAPTER::createLayers( REPORTER* aStatusReporter )
         if( aStatusReporter )
             aStatusReporter->Report( _( "Create zones" ) );
 
-        std::vector<std::pair<const ZONE*, PCB_LAYER_ID>> zones;
+        std::vector<std::pair<ZONE*, PCB_LAYER_ID>> zones;
+        std::unordered_map<PCB_LAYER_ID, std::unique_ptr<std::mutex>> layer_lock;
 
         for( ZONE* zone : m_board->Zones() )
         {
             for( PCB_LAYER_ID layer : zone->GetLayerSet().Seq() )
+            {
                 zones.emplace_back( std::make_pair( zone, layer ) );
+                layer_lock.emplace( layer, std::make_unique<std::mutex>() );
+            }
         }
 
         // Add zones objects
         std::atomic<size_t> nextZone( 0 );
         std::atomic<size_t> threadsFinished( 0 );
 
-        size_t parallelThreadCount = std::max<size_t>( std::thread::hardware_concurrency(), 2 );
+        size_t parallelThreadCount = std::min<size_t>( zones.size(),
+                std::max<size_t>( std::thread::hardware_concurrency(), 2 ) );
 
         for( size_t ii = 0; ii < parallelThreadCount; ++ii )
         {
@@ -708,7 +713,7 @@ void BOARD_ADAPTER::createLayers( REPORTER* aStatusReporter )
                             areaId < zones.size();
                             areaId = nextZone.fetch_add( 1 ) )
                 {
-                    const ZONE* zone = zones[areaId].first;
+                    ZONE* zone = zones[areaId].first;
 
                     if( zone == nullptr )
                         break;
@@ -716,9 +721,22 @@ void BOARD_ADAPTER::createLayers( REPORTER* aStatusReporter )
                     PCB_LAYER_ID layer = zones[areaId].second;
 
                     auto layerContainer = m_layerMap.find( layer );
+                    auto layerPolyContainer = m_layers_poly.find( layer );
 
                     if( layerContainer != m_layerMap.end() )
+                    {
                         addSolidAreasShapes( zone, layerContainer->second, layer );
+                    }
+
+                    if( GetFlag( FL_RENDER_OPENGL_COPPER_THICKNESS )
+                      && ( m_renderEngine == RENDER_ENGINE::OPENGL_LEGACY )
+                      && layerPolyContainer != m_layers_poly.end() )
+                    {
+                        auto mut_it = layer_lock.find( layer );
+
+                        std::lock_guard< std::mutex > lock( *( mut_it->second ) );
+                        zone->TransformSolidAreasShapesToPolygon( layer, *layerPolyContainer->second );
+                    }
                 }
 
                 threadsFinished++;
@@ -732,25 +750,6 @@ void BOARD_ADAPTER::createLayers( REPORTER* aStatusReporter )
 
     }
 
-    if( GetFlag( FL_ZONE ) && GetFlag( FL_RENDER_OPENGL_COPPER_THICKNESS )
-      && ( m_renderEngine == RENDER_ENGINE::OPENGL_LEGACY ) )
-    {
-        // Add copper zones
-        for( ZONE* zone : m_board->Zones() )
-        {
-            if( zone == nullptr )
-                break;
-
-            for( PCB_LAYER_ID layer : zone->GetLayerSet().Seq() )
-            {
-                auto layerContainer = m_layers_poly.find( layer );
-
-                if( layerContainer != m_layers_poly.end() )
-                    zone->TransformSolidAreasShapesToPolygon( layer, *layerContainer->second );
-            }
-        }
-    }
-
     // Simplify layer polygons
 
     if( aStatusReporter )