diff --git a/fusion_plating/fusion_plating_configurator/models/fp_part_catalog.py b/fusion_plating/fusion_plating_configurator/models/fp_part_catalog.py
index cf2049ee..c2e6b0cc 100644
--- a/fusion_plating/fusion_plating_configurator/models/fp_part_catalog.py
+++ b/fusion_plating/fusion_plating_configurator/models/fp_part_catalog.py
@@ -131,6 +131,21 @@ class FpPartCatalog(models.Model):
     notes = fields.Html(string='Notes')
     active = fields.Boolean(string='Active', default=True)
 
+    # ---- Direct-order defaults (Phase C — C4) ----
+    x_fc_default_coating_config_id = fields.Many2one(
+        'fp.coating.config',
+        string='Default Treatment',
+        help='Default coating applied when this part is dropped onto a '
+             'direct order line. Updated when "Save as Default" is ticked.',
+    )
+    x_fc_default_treatment_ids = fields.Many2many(
+        'fp.treatment',
+        relation='fp_part_catalog_default_treatment_rel',
+        string='Default Additional Treatments',
+        help='Default additional treatments. Seeded when "Save as Default" '
+             'is ticked on a direct order line.',
+    )
+
     # Substrate density mapping (g/cm³) for material weight calculation
     _SUBSTRATE_DENSITY = {
         'aluminium': 2.70,
diff --git a/fusion_plating/fusion_plating_configurator/views/fp_part_catalog_views.xml b/fusion_plating/fusion_plating_configurator/views/fp_part_catalog_views.xml
index 488f960f..d0bc4b48 100644
--- a/fusion_plating/fusion_plating_configurator/views/fp_part_catalog_views.xml
+++ b/fusion_plating/fusion_plating_configurator/views/fp_part_catalog_views.xml
@@ -224,6 +224,20 @@
                                 </list>
                             </field>
                         </page>
+                        <page string="Defaults" name="direct_order_defaults">
+                            <group>
+                                <field name="x_fc_default_coating_config_id"
+                                       options="{'no_create_edit': True}"/>
+                                <field name="x_fc_default_treatment_ids"
+                                       widget="many2many_tags"
+                                       options="{'no_create_edit': True}"/>
+                            </group>
+                            <p class="text-muted">
+                                Seeds the treatment fields on new direct-order
+                                lines. Updated whenever "Save as Default" is
+                                ticked while placing an order.
+                            </p>
+                        </page>
                         <page string="Notes" name="notes">
                             <field name="notes" placeholder="Additional notes about this part..."/>
                         </page>
diff --git a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_line.py b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_line.py
index d60bf854..a37edd9b 100644
--- a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_line.py
+++ b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_line.py
@@ -103,6 +103,11 @@ class FpDirectOrderLine(models.TransientModel):
         help='Do not save this as a reusable part in the catalog after the '
              'order is created. Useful for quote-only or prototype parts.',
     )
+    push_to_defaults = fields.Boolean(
+        string='Save as Default',
+        help='After submit, write this line\'s coating + additional '
+             'treatments back onto the part catalog as its new defaults.',
+    )
 
     # ---- Description ----
     description_template_id = fields.Many2one(
@@ -138,6 +143,16 @@ class FpDirectOrderLine(models.TransientModel):
             )
 
     # ---- Onchange ----
+    @api.onchange('part_catalog_id')
+    def _onchange_part_defaults(self):
+        """When a part is picked, seed coating + treatments from its catalog defaults."""
+        if not self.part_catalog_id:
+            return
+        if not self.coating_config_id and self.part_catalog_id.x_fc_default_coating_config_id:
+            self.coating_config_id = self.part_catalog_id.x_fc_default_coating_config_id
+        if not self.treatment_ids and self.part_catalog_id.x_fc_default_treatment_ids:
+            self.treatment_ids = self.part_catalog_id.x_fc_default_treatment_ids
+
     @api.onchange('coating_config_id', 'quantity', 'part_catalog_id')
     def _onchange_lookup_price(self):
         """Auto-fill unit_price from customer price list when available."""
diff --git a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard.py b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard.py
index 2a64740d..c1bb5204 100644
--- a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard.py
+++ b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard.py
@@ -251,6 +251,20 @@ class FpDirectOrderWizard(models.TransientModel):
         # 5. Create + confirm
         so = self.env['sale.order'].create(so_vals)
         so.action_confirm()
+
+        # 6. Push-to-defaults (C4) — after the part has been resolved /
+        # revision-bumped, write coating + treatments back onto the part
+        # catalog entry so the next order inherits the same defaults.
+        for line in self.line_ids:
+            if not line.push_to_defaults:
+                continue
+            part = line.part_catalog_id
+            if not part or line.is_one_off:
+                continue
+            part.write({
+                'x_fc_default_coating_config_id': line.coating_config_id.id or False,
+                'x_fc_default_treatment_ids': [(6, 0, line.treatment_ids.ids)],
+            })
         so.message_post(body=_(
             'Direct order created from PO %s with %d line(s). '
             'Quotation stage skipped.'
diff --git a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard_views.xml b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard_views.xml
index ac2717d0..d79f4929 100644
--- a/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard_views.xml
+++ b/fusion_plating/fusion_plating_configurator/wizard/fp_direct_order_wizard_views.xml
@@ -156,6 +156,8 @@
                                         <field name="start_at_node_id"
                                                options="{'no_create': True}"/>
                                         <field name="is_one_off"/>
+                                        <field name="push_to_defaults"
+                                               invisible="is_one_off"/>
                                     </group>
                                 </form>
                             </field>