diff --git a/OpenNest.WinForms.Tests/Forms/BomImportFormTests.cs b/OpenNest.WinForms.Tests/Forms/BomImportFormTests.cs
new file mode 100644
index 0000000..6357dc7
--- /dev/null
+++ b/OpenNest.WinForms.Tests/Forms/BomImportFormTests.cs
@@ -0,0 +1,188 @@
+using System.Data;
+using System.Reflection;
+using System.Windows.Forms;
+using OpenNest.Forms;
+using OpenNest.IO.Bom;
+
+namespace OpenNest.WinForms.Tests.Forms;
+
+public class BomImportFormTests
+{
+    [Fact]
+    public void PartColumns_KeepBomOrderAndOnlyMaterialThicknessAndQtyAreEditable()
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+
+            Assert.Equal(
+                new[] { "Item #", "File Name", "Description", "Material", "Thickness", "Qty", "Status" },
+                parts.Columns.Cast<DataGridViewColumn>().OrderBy(c => c.DisplayIndex).Select(c => c.HeaderText)
+            );
+            Assert.All(
+                parts.Columns.Cast<DataGridViewColumn>(),
+                c => Assert.Equal(DataGridViewColumnSortMode.NotSortable, c.SortMode)
+            );
+            Assert.Equal(
+                new[] { "colMaterial", "colThickness", "colQty" },
+                parts.Columns.Cast<DataGridViewColumn>().Where(c => !c.ReadOnly).Select(c => c.Name)
+            );
+        });
+    }
+
+    [Fact]
+    public void EditingQty_ChangesThatPartAndTheGroupTotal()
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+            var first = Ready("PT01", qty: 2);
+            var second = Ready("PT02", qty: 3);
+            form.LoadRows(new[] { first, second });
+            Assert.Equal(5, GroupTotal(form, 0));
+
+            Assert.True(Edit(parts, 0, "colQty", "5"));
+
+            Assert.Equal(5, first.Qty);
+            Assert.Equal(3, second.Qty);
+            Assert.Equal(8, GroupTotal(form, 0));
+            Assert.Equal("Ready", parts.Rows[0].Cells["colStatus"].Value);
+        });
+    }
+
+    [Theory]
+    [InlineData("0")]
+    [InlineData("-1")]
+    [InlineData("2.5")]
+    [InlineData("abc")]
+    [InlineData("")]
+    public void InvalidQty_IsRefusedAndTheOldValueKept(string text)
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+            var part = Ready("PT01", qty: 2);
+            form.LoadRows(new[] { part });
+
+            Assert.False(Edit(parts, 0, "colQty", text));
+            Assert.True(parts.IsCurrentCellInEditMode);
+            Assert.False(string.IsNullOrEmpty(parts.CurrentCell.ErrorText));
+
+            parts.CancelEdit();
+            Assert.True(parts.EndEdit());
+
+            Assert.Equal(2, part.Qty);
+            Assert.Equal(2, GroupTotal(form, 0));
+        });
+    }
+
+    [Fact]
+    public void Edits_ChangeThePartShownInThatRow_AfterTheRowsAreReloadedInAnotherOrder()
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+            var a = Ready("PT01", qty: 1);
+            var b = Ready("PT02", qty: 1);
+            var c = Ready("PT03", qty: 1);
+            form.LoadRows(new[] { a, b, c });
+
+            Assert.True(Edit(parts, 2, "colMaterial", "Aluminum"));
+            Assert.Equal("Aluminum", c.Material);
+            Assert.Equal("Stainless", a.Material);
+
+            form.LoadRows(new[] { c, a, b });
+            Assert.Same(c, parts.Rows[0].DataBoundItem);
+
+            Assert.True(Edit(parts, 0, "colQty", "9"));
+            Assert.Equal(9, c.Qty);
+            Assert.Equal(1, a.Qty);
+        });
+    }
+
+    [Fact]
+    public void RowsWithoutADrawing_CannotEnterEditMode()
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+            var noDrawing = Ready("PT01", qty: 1);
+            noDrawing.DxfPath = null;
+            form.LoadRows(new[] { noDrawing, Ready("PT02", qty: 1) });
+
+            parts.CurrentCell = parts.Rows[0].Cells["colQty"];
+            Assert.False(parts.BeginEdit(selectAll: true));
+            Assert.False(parts.IsCurrentCellInEditMode);
+
+            parts.CurrentCell = parts.Rows[1].Cells["colQty"];
+            Assert.True(parts.BeginEdit(selectAll: true));
+            parts.CancelEdit();
+        });
+    }
+
+    [Fact]
+    public void BlankingMaterialOrThickness_MarksTheRowAndDropsItFromTheGroups()
+    {
+        RunSta(() =>
+        {
+            using var form = Show(out var parts);
+            var first = Ready("PT01", qty: 1);
+            var second = Ready("PT02", qty: 1);
+            form.LoadRows(new[] { first, second });
+
+            Assert.True(Edit(parts, 0, "colMaterial", " "));
+            Assert.Null(first.Material);
+            Assert.Equal("Needs material", parts.Rows[0].Cells["colStatus"].Value);
+
+            Assert.False(Edit(parts, 1, "colThickness", "abc"));
+            parts.CancelEdit();
+            Assert.True(parts.EndEdit());
+            Assert.Equal(0.25, second.Thickness);
+
+            Assert.True(Edit(parts, 1, "colThickness", ""));
+            Assert.Null(second.Thickness);
+            Assert.Equal("Needs thickness", parts.Rows[1].Cells["colStatus"].Value);
+
+            Assert.Empty(GroupsTable(form).Rows);
+            Assert.Equal("0 ready, 1 needs a material, 1 needs a thickness", Field<Label>(form, "lblSummary").Text);
+        });
+    }
+
+    private static BomImportForm Show(out DataGridView parts)
+    {
+        var form = new BomImportForm();
+        form.Show();
+        parts = Field<DataGridView>(form, "dgvParts");
+        return form;
+    }
+
+    private static bool Edit(DataGridView grid, int row, string column, string text)
+    {
+        grid.CurrentCell = grid.Rows[row].Cells[column];
+        Assert.True(grid.BeginEdit(selectAll: true));
+        grid.EditingControl!.Text = text;
+        return grid.EndEdit();
+    }
+
+    private static DataTable GroupsTable(BomImportForm form) =>
+        Assert.IsType<DataTable>(Field<DataGridView>(form, "dgvGroups").DataSource);
+
+    private static int GroupTotal(BomImportForm form, int group) =>
+        (int)GroupsTable(form).Rows[group]["Total Qty"];
+
+    private static BomPartRow Ready(string fileName, int qty) =>
+        new()
+        {
+            FileName = fileName,
+            DxfPath = $@"C:\drawings\{fileName}.dxf",
+            Material = "Stainless",
+            Thickness = 0.25,
+            Qty = qty,
+        };
+
+    private static T Field<T>(object owner, string name) =>
+        (T)owner.GetType().GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!.GetValue(owner)!;
+
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(30), "The BOM import dialog test did not complete.");
+}
diff --git a/OpenNest/Forms/BomImportForm.Designer.cs b/OpenNest/Forms/BomImportForm.Designer.cs
index 1361777..37fb528 100644
--- a/OpenNest/Forms/BomImportForm.Designer.cs
+++ b/OpenNest/Forms/BomImportForm.Designer.cs
@@ -15,6 +15,8 @@ namespace OpenNest.Forms
 
         private void InitializeComponent()
         {
+            System.Windows.Forms.DataGridViewCellStyle thicknessCellStyle = new System.Windows.Forms.DataGridViewCellStyle();
+            System.Windows.Forms.DataGridViewCellStyle qtyCellStyle = new System.Windows.Forms.DataGridViewCellStyle();
             grpInput = new System.Windows.Forms.GroupBox();
             tbl = new System.Windows.Forms.TableLayoutPanel();
             lblJobName = new System.Windows.Forms.Label();
@@ -34,6 +36,13 @@ namespace OpenNest.Forms
             tabControl = new System.Windows.Forms.TabControl();
             tabParts = new System.Windows.Forms.TabPage();
             dgvParts = new System.Windows.Forms.DataGridView();
+            colItemNum = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colFileName = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colDescription = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colMaterial = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colThickness = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colQty = new System.Windows.Forms.DataGridViewTextBoxColumn();
+            colStatus = new System.Windows.Forms.DataGridViewTextBoxColumn();
             tabGroups = new System.Windows.Forms.TabPage();
             dgvGroups = new System.Windows.Forms.DataGridView();
             pnlBottom = new System.Windows.Forms.Panel();
@@ -265,11 +274,84 @@ namespace OpenNest.Forms
             dgvParts.BackgroundColor = System.Drawing.SystemColors.Window;
             dgvParts.BorderStyle = System.Windows.Forms.BorderStyle.None;
             dgvParts.ColumnHeadersHeightSizeMode = System.Windows.Forms.DataGridViewColumnHeadersHeightSizeMode.AutoSize;
+            dgvParts.Columns.AddRange(new System.Windows.Forms.DataGridViewColumn[] { colItemNum, colFileName, colDescription, colMaterial, colThickness, colQty, colStatus });
             dgvParts.Dock = System.Windows.Forms.DockStyle.Fill;
             dgvParts.Name = "dgvParts";
             dgvParts.RowHeadersVisible = false;
             dgvParts.SelectionMode = System.Windows.Forms.DataGridViewSelectionMode.FullRowSelect;
             dgvParts.TabIndex = 0;
+            dgvParts.CellBeginEdit += DgvParts_CellBeginEdit;
+            dgvParts.CellEndEdit += DgvParts_CellEndEdit;
+            dgvParts.CellFormatting += DgvParts_CellFormatting;
+            dgvParts.CellParsing += DgvParts_CellParsing;
+            dgvParts.CellToolTipTextNeeded += DgvParts_CellToolTipTextNeeded;
+            dgvParts.CellValidating += DgvParts_CellValidating;
+            dgvParts.DataError += DgvParts_DataError;
+            //
+            // colItemNum
+            //
+            colItemNum.DataPropertyName = "ItemNum";
+            colItemNum.FillWeight = 40F;
+            colItemNum.HeaderText = "Item #";
+            colItemNum.Name = "colItemNum";
+            colItemNum.ReadOnly = true;
+            colItemNum.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colFileName
+            //
+            colFileName.DataPropertyName = "FileName";
+            colFileName.FillWeight = 120F;
+            colFileName.HeaderText = "File Name";
+            colFileName.Name = "colFileName";
+            colFileName.ReadOnly = true;
+            colFileName.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colDescription
+            //
+            colDescription.DataPropertyName = "Description";
+            colDescription.FillWeight = 150F;
+            colDescription.HeaderText = "Description";
+            colDescription.Name = "colDescription";
+            colDescription.ReadOnly = true;
+            colDescription.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colMaterial
+            //
+            colMaterial.DataPropertyName = "Material";
+            colMaterial.FillWeight = 110F;
+            colMaterial.HeaderText = "Material";
+            colMaterial.Name = "colMaterial";
+            colMaterial.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colThickness
+            //
+            thicknessCellStyle.Alignment = System.Windows.Forms.DataGridViewContentAlignment.MiddleRight;
+            thicknessCellStyle.Format = "0.####";
+            colThickness.DataPropertyName = "Thickness";
+            colThickness.DefaultCellStyle = thicknessCellStyle;
+            colThickness.FillWeight = 70F;
+            colThickness.HeaderText = "Thickness";
+            colThickness.Name = "colThickness";
+            colThickness.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colQty
+            //
+            qtyCellStyle.Alignment = System.Windows.Forms.DataGridViewContentAlignment.MiddleRight;
+            colQty.DataPropertyName = "Qty";
+            colQty.DefaultCellStyle = qtyCellStyle;
+            colQty.FillWeight = 50F;
+            colQty.HeaderText = "Qty";
+            colQty.Name = "colQty";
+            colQty.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
+            //
+            // colStatus
+            //
+            colStatus.DataPropertyName = "StatusText";
+            colStatus.FillWeight = 100F;
+            colStatus.HeaderText = "Status";
+            colStatus.Name = "colStatus";
+            colStatus.ReadOnly = true;
+            colStatus.SortMode = System.Windows.Forms.DataGridViewColumnSortMode.NotSortable;
             //
             // tabGroups
             //
@@ -383,6 +465,13 @@ namespace OpenNest.Forms
         private System.Windows.Forms.TabControl tabControl;
         private System.Windows.Forms.TabPage tabParts;
         private System.Windows.Forms.DataGridView dgvParts;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colItemNum;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colFileName;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colDescription;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colMaterial;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colThickness;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colQty;
+        private System.Windows.Forms.DataGridViewTextBoxColumn colStatus;
         private System.Windows.Forms.TabPage tabGroups;
         private System.Windows.Forms.DataGridView dgvGroups;
         private System.Windows.Forms.Panel pnlBottom;
diff --git a/OpenNest/Forms/BomImportForm.cs b/OpenNest/Forms/BomImportForm.cs
index 183ae69..88e7cd7 100644
--- a/OpenNest/Forms/BomImportForm.cs
+++ b/OpenNest/Forms/BomImportForm.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.IO;
@@ -12,9 +13,10 @@ namespace OpenNest.Forms
 {
     public partial class BomImportForm : Form
     {
-        private List<BomPartRow> _parts;
+        private static readonly Color NeedsInputBackColor = Color.FromArgb(255, 240, 200);
+
+        private readonly BindingList<BomPartRow> _parts;
         private Dictionary<string, BomGroupPlateSettings> _groupSettings;
-        private bool _suppressRegroup;
         private NestDefaults _defaults;
 
         public Form MdiParentForm { get; set; }
@@ -22,7 +24,10 @@ namespace OpenNest.Forms
         public BomImportForm()
         {
             InitializeComponent();
-            _parts = new List<BomPartRow>();
+            _parts = new BindingList<BomPartRow>();
+            _parts.ListChanged += Parts_ListChanged;
+            dgvParts.AutoGenerateColumns = false;
+            dgvParts.DataSource = _parts;
             _groupSettings = new Dictionary<string, BomGroupPlateSettings>();
             _defaults = MainForm.LoadSavedNestDefaults(out _);
             ApplyDefaults();
@@ -99,11 +104,7 @@ namespace OpenNest.Forms
                 using (var reader = new BomReader(txtBomFile.Text))
                     items = reader.GetItems();
 
-                _parts = BomImportRows.Build(items, txtDxfFolder.Text);
-                _groupSettings.Clear();
-                PopulatePartsGrid();
-                RebuildGroups();
-                UpdateSummary();
+                LoadRows(BomImportRows.Build(items, txtDxfFolder.Text));
                 btnCreateNests.Enabled = true;
                 tabControl.SelectedTab = tabParts;
             }
@@ -122,87 +123,133 @@ namespace OpenNest.Forms
 
         #region Parts Tab
 
-        private void PopulatePartsGrid()
+        /// <summary>
+        /// Shows <paramref name="rows"/> in the Parts table, in their order, and
+        /// rebuilds the groups and summary from them.
+        /// </summary>
+        internal void LoadRows(IEnumerable<BomPartRow> rows)
         {
-            _suppressRegroup = true;
-
-            var table = new DataTable();
-            table.Columns.Add("Item #", typeof(string));
-            table.Columns.Add("File Name", typeof(string));
-            table.Columns.Add("Qty", typeof(string));
-            table.Columns.Add("Description", typeof(string));
-            table.Columns.Add("Material", typeof(string));
-            table.Columns.Add("Thickness", typeof(string));
-            table.Columns.Add("Status", typeof(string));
-
-            foreach (var part in _parts)
-            {
-                table.Rows.Add(
-                    part.ItemNum?.ToString() ?? "",
-                    part.FileName ?? "",
-                    part.Qty?.ToString() ?? "",
-                    part.Description ?? "",
-                    part.Material ?? "",
-                    part.Thickness?.ToString("0.####") ?? "",
-                    part.StatusText
-                );
-            }
-
-            dgvParts.DataSource = table;
-
-            // Make non-editable columns read-only
-            foreach (DataGridViewColumn col in dgvParts.Columns)
-            {
-                if (col.Name != "Material" && col.Name != "Thickness")
-                    col.ReadOnly = true;
-            }
-
-            // Style rows by status
-            for (var i = 0; i < _parts.Count; i++)
-            {
-                if (!_parts[i].IsEditable)
-                {
-                    dgvParts.Rows[i].ReadOnly = true;
-                    dgvParts.Rows[i].DefaultCellStyle.ForeColor = Color.Gray;
-                }
-            }
-
-            dgvParts.CellValueChanged -= DgvParts_CellValueChanged;
-            dgvParts.CellValueChanged += DgvParts_CellValueChanged;
-
-            _suppressRegroup = false;
-        }
-
-        private void DgvParts_CellValueChanged(object sender, DataGridViewCellEventArgs e)
-        {
-            if (_suppressRegroup || e.RowIndex < 0)
-                return;
-
-            var colName = dgvParts.Columns[e.ColumnIndex].Name;
-            if (colName != "Material" && colName != "Thickness")
-                return;
-
-            var part = _parts[e.RowIndex];
-            if (!part.IsEditable)
-                return;
-
-            if (colName == "Material")
-                part.Material = dgvParts.Rows[e.RowIndex].Cells[e.ColumnIndex].Value?.ToString();
-
-            if (colName == "Thickness")
-            {
-                var text = dgvParts.Rows[e.RowIndex].Cells[e.ColumnIndex].Value?.ToString();
-                part.Thickness = double.TryParse(text, out var t) ? t : (double?)null;
-            }
-
-            _suppressRegroup = true;
-            dgvParts.Rows[e.RowIndex].Cells["Status"].Value = part.StatusText;
-            _suppressRegroup = false;
+            _groupSettings.Clear();
+            _parts.RaiseListChangedEvents = false;
+            _parts.Clear();
+            foreach (var row in rows)
+                _parts.Add(row);
+            _parts.RaiseListChangedEvents = true;
+            _parts.ResetBindings();
 
             RebuildGroups();
             UpdateSummary();
         }
 
+        private BomPartRow PartAt(int rowIndex) =>
+            rowIndex >= 0 && rowIndex < dgvParts.Rows.Count
+                ? dgvParts.Rows[rowIndex].DataBoundItem as BomPartRow
+                : null;
+
+        private void Parts_ListChanged(object sender, ListChangedEventArgs e)
+        {
+            if (e.ListChangedType != ListChangedType.ItemChanged)
+                return;
+
+            switch (e.PropertyDescriptor?.Name)
+            {
+                case nameof(BomPartRow.Material):
+                case nameof(BomPartRow.Thickness):
+                case nameof(BomPartRow.Qty):
+                    RebuildGroups();
+                    UpdateSummary();
+                    break;
+            }
+        }
+
+        private void DgvParts_CellBeginEdit(object sender, DataGridViewCellCancelEventArgs e)
+        {
+            // Rows without a drawing can never be imported, so they stay locked.
+            if (PartAt(e.RowIndex) is not { IsEditable: true })
+                e.Cancel = true;
+        }
+
+        private void DgvParts_CellValidating(object sender, DataGridViewCellValidatingEventArgs e)
+        {
+            if (!dgvParts.IsCurrentCellInEditMode)
+                return;
+
+            var text = e.FormattedValue?.ToString();
+            string error = null;
+
+            if (e.ColumnIndex == colQty.Index && !BomQuantity.TryParse(text, out _))
+                error = "Enter a whole number of 1 or more.";
+            else if (
+                e.ColumnIndex == colThickness.Index
+                && !string.IsNullOrWhiteSpace(text)
+                && !TryParseThickness(text, out _)
+            )
+                error = "Enter a thickness greater than 0, or leave it blank.";
+
+            dgvParts.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = error ?? "";
+            if (error != null)
+                e.Cancel = true;
+        }
+
+        private void DgvParts_CellParsing(object sender, DataGridViewCellParsingEventArgs e)
+        {
+            var text = e.Value?.ToString();
+
+            if (e.ColumnIndex == colQty.Index && BomQuantity.TryParse(text, out var qty))
+            {
+                e.Value = qty;
+                e.ParsingApplied = true;
+            }
+            else if (e.ColumnIndex == colThickness.Index)
+            {
+                e.Value = TryParseThickness(text, out var thickness) ? thickness : null;
+                e.ParsingApplied = true;
+            }
+            else if (e.ColumnIndex == colMaterial.Index)
+            {
+                e.Value = string.IsNullOrWhiteSpace(text) ? null : text;
+                e.ParsingApplied = true;
+            }
+        }
+
+        private void DgvParts_CellEndEdit(object sender, DataGridViewCellEventArgs e)
+        {
+            dgvParts.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = "";
+        }
+
+        private void DgvParts_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
+        {
+            var part = PartAt(e.RowIndex);
+            if (part == null)
+                return;
+
+            if (!part.IsEditable)
+                e.CellStyle.ForeColor = Color.Gray;
+            else if (part.Status != BomRowStatus.Ready)
+                e.CellStyle.BackColor = NeedsInputBackColor;
+        }
+
+        private void DgvParts_CellToolTipTextNeeded(
+            object sender,
+            DataGridViewCellToolTipTextNeededEventArgs e
+        )
+        {
+            if (e.ColumnIndex == colQty.Index && PartAt(e.RowIndex) is { QtyAssumed: true })
+                e.ToolTipText = "The BOM has no quantity for this part; 1 is used.";
+        }
+
+        private void DgvParts_DataError(object sender, DataGridViewDataErrorEventArgs e)
+        {
+            // Never show the grid's default error dialog; keep the edit open instead.
+            e.ThrowException = false;
+            e.Cancel = true;
+            if (e.RowIndex >= 0 && e.ColumnIndex >= 0)
+                dgvParts.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = "This value cannot be used.";
+        }
+
+        private static bool TryParseThickness(string text, out double thickness) =>
+            double.TryParse(text, out thickness) && double.IsFinite(thickness) && thickness > 0;
+
         #endregion
 
         #region Groups Tab
@@ -319,7 +366,7 @@ namespace OpenNest.Forms
 
         private void CreateNests_Click(object sender, EventArgs e)
         {
-            if (_parts == null || _parts.Count == 0)
+            if (_parts.Count == 0 || !dgvParts.EndEdit())
                 return;
 
             // Save latest group edits