🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Panel Repair and Replacement › Lesson 10
TRN-AUTO25-03 · Lesson 10 · 16.5 clock hours · practice

Glaze, Block Sanding, and Substrate Finishing

Objective. Students will apply polyester spot putty or glaze coat to fill pinholes and low spots, then block-sand through a progressive grit sequence to produce a substrate profile ready for primer-surfacer.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Glaze Coat Exists

Body filler and glaze coat do different jobs. Body filler is a thick polyester compound designed to fill deep dents, low areas, and surface voids — it builds significant thickness and is spread in layers. Glaze coat (also called spot putty or polyester spot putty) is a fine-grained, low-viscosity material made to fill only pinholes, sand scratches, and small low spots that body filler left behind. Glaze coat has very little build capability — applying it thick causes it to shrink, crack, or fail adhesion. Think of filler as the structural layer and glaze as the finishing layer: each has its place in the repair sequence and neither substitutes for the other.

Reading the Panel Before You Apply Glaze

Before reaching for glaze, inspect the panel under raking light — a light source held low and nearly parallel to the surface. Raking light reveals pinholes, scratches, and low spots that look invisible under flat overhead lighting. Mark any pinhole clusters or low spots with a grease pencil so you do not miss them during application. Wipe the entire panel with wax-and-grease remover on a clean lint-free cloth and allow it to flash dry. Contamination under glaze causes adhesion failure and will show up as lifting or bubbling during primer cure.

Mixing and Applying Glaze Coat

Polyester glaze coat is catalyzed the same way as body filler — a small ribbon of hardener is folded into the glaze on a clean mixing board using a plastic spreader. Follow the manufacturer's ratio guidance on the product label; too little hardener leaves the glaze soft and tacky, too much causes rapid hardening and air entrapment. Mix thoroughly but avoid whipping air into the material, because air introduces the pinholes you are trying to eliminate.

Apply glaze in thin skim coats only — two or three light passes over the problem area, not a thick buildup. Hold the spreader at roughly 60–75 degrees to the panel and draw it smoothly in one direction. Work quickly because pot life is short at shop temperature. Allow the glaze to cure fully (it should feel firm and not drag under a fingernail) before sanding. Sanding soft glaze smears it into surrounding scratches rather than cutting it flat.

Guide Coat and the Grit Sequence

A guide coat is a thin, contrasting color applied over the cured surface before each sanding stage. Aerosol dry-guide-coat products are common in modern shops. The guide coat does one thing: it shows you exactly where your block has and has not touched the surface. Low spots retain guide coat while high spots cut through quickly. Never skip the guide coat — it is the only reliable way to confirm flatness without special equipment.

The grit sequence used in this lesson follows a logical progression that removes scratches from each previous stage:

Never skip grits. Jumping from 80 to 220, for example, leaves coarse scratches that primer-surfacer cannot fill, and they telegraph through the finished paint as visible lines.

Block-Sanding Technique

A sanding block — rigid for flat panels, flexible for gentle curves — keeps the paper flat on the surface and prevents fingertip low spots. Hold the block with fingers spread across its full length. Use straight, overlapping strokes that cross the previous direction by about 45 degrees. Apply even, moderate pressure — let the paper do the cutting. Excessive pressure causes the block to flex around high spots and skip low ones. Keep strokes long and consistent across the full panel section rather than working in small circles.

A dual-action (DA) sander can be used for bulk material removal with 80 or 150 grit, but the final straightness passes at 180 and 220 grit must be done by hand with a block to feel and correct high-low variation.

Straightness Check

After completing 220-grit block sanding, use a 600 mm metal straightedge laid across the panel in at least three directions — horizontally, vertically, and diagonally. Slide a feeler gauge under the straightedge at multiple points. Any gap greater than 1.5 mm indicates a low spot that must be addressed before the panel can be signed off and advanced to primer-surfacer. Document your check on your work order.

Safety at This Stage

Dry sanding generates fine polyester dust that is harmful when inhaled. Wear a properly fitted NIOSH-approved dust respirator (not just a paper dust mask) any time you sand body filler or glaze. Safety glasses protect eyes from airborne particles. Keep the work area ventilated. Wax-and-grease remover solvents are flammable — keep containers capped when not in use and away from ignition sources. If you use a DA sander, confirm the guard is in place and the power cord or air hose is clear of the sanding path before starting.

Key terms9

Glaze coat (spot putty)A fine-grained polyester material used to fill pinholes, sand scratches, and small low spots left after body filler; not intended for thick buildup.
Raking lightA light source positioned low and nearly parallel to the panel surface to reveal surface defects such as pinholes, low spots, and sanding scratches that are invisible under flat overhead lighting.
Guide coatA thin contrasting coating applied over a sanded surface before the next sanding stage; areas that retain guide coat after sanding identify low spots that the block has not reached.
Grit sequenceThe ordered progression of sandpaper grits (e.g., 80 → 150 → 180 → 220) where each stage removes scratches from the previous coarser grit; skipping grits leaves scratches too deep for subsequent stages to remove.
BlockingHand-sanding technique in which sandpaper is backed by a rigid or flexible block to keep the abrasive flat on the surface and prevent uneven pressure from fingertips.
Feeler gaugeA set of precision metal blades of known thickness used to measure the gap between a straightedge and a panel surface to quantify surface deviation.
Pot lifeThe working time available after a catalyzed material such as glaze coat or body filler is mixed before it becomes too stiff to apply properly.
Wax-and-grease removerA solvent-based cleaner applied with a lint-free cloth to remove surface contamination before any filler, glaze, or coating is applied; essential for adhesion.
Primer-surfacerA high-build primer applied over a properly prepared substrate to fill fine scratches and provide a uniform base for topcoats; requires an adequately refined substrate profile to perform correctly.

Worked examples4

1. After block-sanding with 150 grit and applying guide coat, you lay the straightedge diagonally across the panel and slide a feeler gauge under it. The gauge slides freely at the 1.2 mm blade but stops at the 1.6 mm blade. Does this area pass the 1.5 mm flatness check?
  1. The acceptance criterion is a gap no greater than 1.5 mm.
  2. The 1.2 mm blade passes freely, confirming the gap is at least 1.2 mm.
  3. The 1.6 mm blade does not pass, confirming the gap is less than 1.6 mm.
  4. Therefore the gap is between 1.2 mm and 1.6 mm — it could be anywhere in that range, including above 1.5 mm.
  5. Because you cannot confirm the gap is within 1.5 mm (the 1.5 mm blade was not tested), the safe and correct decision is to test with the 1.5 mm blade.
  6. If the 1.5 mm blade also slides freely, the gap exceeds 1.5 mm and the area fails. If the 1.5 mm blade does not pass, the gap is under 1.5 mm and the area passes.
Answer: Additional measurement with the 1.5 mm blade is required. If it passes freely, the deviation exceeds 1.5 mm and the panel fails — additional glaze and block-sanding are needed before sign-off.
2. You have a panel 500 mm wide. Your sanding block is 70 mm wide. To ensure complete coverage with 50% overlap on each pass, how many parallel strokes are needed to cover the full width once?
  1. With 50% overlap, each stroke advances the block by half its width: 70 mm ÷ 2 = 35 mm effective advance per stroke.
  2. Number of strokes = panel width ÷ effective advance = 500 mm ÷ 35 mm = 14.3.
  3. Round up to the nearest whole stroke to ensure full coverage: 15 strokes.
Answer: 15 strokes are needed to cover the 500 mm panel width with 50% overlap between each pass.
3. You mix glaze coat and begin applying it, but after 90 seconds the glaze on your spreader is already dragging and not flowing smoothly. The shop temperature is 85 °F (29 °C). What is the most likely cause and what should you do?
  1. Polyester glaze coat cures faster at higher temperatures because heat accelerates the chemical reaction between hardener and resin.
  2. At 85 °F, pot life is significantly shorter than at a cooler shop temperature such as 70 °F.
  3. If the hardener ratio was also on the high end of the recommended range, cure accelerates further.
  4. Action: discard the batch — applying dragging glaze traps air and creates the pinholes you are trying to eliminate.
  5. Mix a fresh, smaller batch and work faster, or reduce hardener slightly within the manufacturer's recommended range. Do not add solvent to extend pot life, as this causes adhesion and shrinkage problems.
Answer: The likely cause is shortened pot life due to elevated shop temperature. Discard the batch, mix a smaller fresh batch, and apply it quickly. Adjust hardener to the lower end of the recommended ratio if the shop remains warm.
4. After 220-grit block sanding, you check the panel under raking light and see a cluster of small dark dots roughly 3 mm in diameter scattered over a 100 mm × 80 mm area. What are these dots and what is the correct next step?
  1. Small dark dots visible under raking light on a sanded glaze surface are pinholes — tiny voids in the glaze created by air bubbles during mixing or application.
  2. 220-grit sanding has opened the tops of the pinholes, making them more visible.
  3. The panel cannot advance to primer-surfacer with visible pinholes — they will telegraph through primer and paint.
  4. Clean the area with wax-and-grease remover and allow to dry.
  5. Mix a small fresh batch of glaze coat and skim it thinly over the pinhole cluster only.
  6. Allow full cure, then re-block-sand through 150 → 180 → 220 grit with guide coat at each stage.
  7. Re-inspect under raking light before sign-off.
Answer: The dots are pinholes. Apply a thin fresh skim coat of glaze over the affected area, cure fully, and re-block-sand through 150, 180, and 220 grit using guide coat at each stage before re-inspecting.
In class: Lecture and demo: difference between body filler and glaze coat — filler builds, glaze fills pinholes only; application technique for thin skim coats; grit sequence logic (80 → 150 → 180 → 220). · Students apply glaze over their lesson 9 panels, focusing only on pinhole and low-spot areas; allow cure and block-sand with a flexible block through 150 and 180 grit, using guide coat at each step. · Final straightness check: students use a long metal straightedge (600 mm) and feeler gauge across the panel in multiple directions; any deviation greater than 1.5 mm requires additional work before the panel is signed off.
Materials: Polyester glaze coat / spot putty, plastic spreaders, dual-action (DA) sander, sanding blocks (flexible and rigid), sandpaper 80–220 grit, guide-coat spray, metal straightedge (600 mm), feeler gauges, wax-and-grease remover, PPE: dust respirator, safety glasses
Check: No pinholes visible under raking light at 220-grit; feeler gauge confirms flatness within 1.5 mm across full panel; student demonstrates correct block-sanding grip and stroke — instructor evaluates under raking light.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Surface Preparation — ASE B2 / I-CAR Panel Repair
1. What is the primary purpose of polyester glaze coat compared to body filler?
Surface Preparation — ASE B2 / I-CAR Refinishing
2. A technician skips 180-grit and sands directly from 150 grit to 220 grit before applying primer-surfacer. What is the most likely result?
Surface Preparation — ASE B2 / I-CAR Panel Repair
3. What does a guide coat tell you during block sanding?
Panel Repair Quality Control — ASE B2 / I-CAR
4. During the straightness check, a feeler gauge blade of 1.5 mm slides freely under the straightedge at one location on a panel. What action should the technician take?
Safety and Hazardous Materials — ASE B2 / I-CAR
5. Why must a technician wear a NIOSH-approved dust respirator when dry-sanding polyester body filler or glaze coat?
Material Application — ASE B2 / I-CAR Panel Repair
6. A technician applies glaze coat in a thick layer to a pinhole area, hoping to fill and level simultaneously. After sanding, the area shows cracking and the glaze separates slightly at the edges. What caused this failure?
Surface Preparation Technique — ASE B2 / I-CAR
7. During block sanding with a rigid block on a flat panel, a technician applies heavy finger pressure at the center of the block. What surface defect is most likely to result?
Tool Selection and Use — ASE B2 / I-CAR Panel Repair
8. At which point in the repair sequence is it correct to use a dual-action (DA) sander rather than hand blocking?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →