diff --git a/image_analysis/structure_refinement/ModelValidation.cpp b/image_analysis/structure_refinement/ModelValidation.cpp
index ab8797727..5e89c63bd 100644
--- a/image_analysis/structure_refinement/ModelValidation.cpp
+++ b/image_analysis/structure_refinement/ModelValidation.cpp
@@ -38,6 +38,7 @@
 #include "../scale_merge/ReindexAmbiguity.h"   // ReindexReflections
 #include "../scale_merge/CrystalSetting.h"     // CellMappingOperators
 #include "ModelFFT.h"
+#include "ModelGrid.h"   // PutModelDensityOnGrid, PutMaskOnGrid
 #include "RigidBodyRefine.h"
 #include "SigmaA.h"
 #ifdef JFJOCH_USE_CUDA
@@ -649,20 +650,20 @@ ModelValidationResult Validate(const std::vector<MergedReflection> &merged,
         dc.set_refmac_compatible_blur(ms.st.models[0]);
 
         // The mask is made on a thread of its own, beside the density: both only read the model, each
-        // fills a grid of its own, and between them they are the whole cost of this step.
+        // fills a grid of its own, and between them they are the whole cost of this step. Both are
+        // gemmi's gridding spread over the threads (ModelGrid.h), the same grids bit for bit at any
+        // thread count; the two grids are the same size, so they share the symmetry's orbits.
         // Refmac radii give a slightly lower R than the Cctbx set on our test cases, at no cost.
-        std::future<gemmi::AsuData<std::complex<float>>> fmask = std::async(std::launch::async,
-                [&model = ms.st.models[0], unit_cell = dc.grid.unit_cell, spacegroup = dc.grid.spacegroup,
-                 spacing = dc.requested_grid_spacing(), to_d] {
-            gemmi::SolventMasker masker(gemmi::AtomicRadiiSet::Refmac);
-            gemmi::Grid<float> mask_grid;
-            mask_grid.unit_cell = unit_cell;
-            mask_grid.spacegroup = spacegroup;
-            mask_grid.set_size_from_spacing(spacing, gemmi::GridSizeRounding::Up);
-            masker.put_mask_on_grid(mask_grid, model);
+        gemmi::Grid<float> mask_grid;
+        mask_grid.unit_cell = dc.grid.unit_cell;
+        mask_grid.spacegroup = dc.grid.spacegroup;
+        mask_grid.set_size_from_spacing(dc.requested_grid_spacing(), gemmi::GridSizeRounding::Up);
+        const std::vector<size_t> orbit_leaders = OrbitLeaders(mask_grid, nthreads);
+        std::future<gemmi::AsuData<std::complex<float>>> fmask = std::async(std::launch::async, [&] {
+            PutMaskOnGrid(mask_grid, ms.st.models[0], orbit_leaders, nthreads);
             return MapToFPhi(mask_grid).prepare_asu_data(to_d, 0);
         });
-        dc.put_model_density_on_grid(ms.st.models[0]);
+        PutModelDensityOnGrid(dc, ms.st.models[0], orbit_leaders, nthreads);
         ms.fcalc = MapToFPhi(dc.grid)
                        .prepare_asu_data(dc.d_min, dc.blur, false, false, false);
         ms.fmask = fmask.get();