A mirror-straight panel is not a paint achievement. By the time colour goes on, the shape is already decided — every wave, every low spot, every ripple in the reflection was set hundreds of hours earlier by somebody with a hammer, a dolly and a sanding board. Paint is a transparent film applied over a surface, and a transparent film cannot hide anything. It reveals.
This is why the most common disappointment in a restoration is a car that looked fine in primer and shows waves in sunlight after paint. Nothing went wrong in the booth. The substrate was never flat, and the primer — matte, light-absorbing, forgiving — was hiding it.
This guide covers how straight bodywork is actually produced: metal finishing before filler, the correct and incorrect use of filler, guide coats, block sanding technique, and setting panel gaps in bare metal. It is the least glamorous phase of a restoration and the one that determines how the finished car looks from twenty feet away.
Get the Metal Right First
The central principle: filler is a levelling compound, not a shaping material. If you are building shape with filler, the metal underneath needs more work.
This is not purism. It is practical. Polyester filler is weaker than steel, it has a different thermal expansion rate, it absorbs moisture over time, and it is only as stable as the surface it is bonded to. A thick section of filler will eventually crack, shrink or telegraph. A skim over properly finished metal will not.
Metal finishing with hammer and dolly
The fundamental skill in bodywork, and it is a skill — it takes practice and the practice should happen on scrap, not on the car.
The basic principle is that you work the metal between a hammer on one side and a dolly on the other. Hammer-on-dolly — striking directly over the dolly — stretches and thins metal, raising a low spot. Hammer-off-dolly — striking next to the dolly while the dolly supports an adjacent high area — lowers a high spot and gathers the metal. Most people use these in exactly the wrong proportion and end up with a panel that is more stretched than when they started.
The common error is overworking. Steel that has been hammered repeatedly in one area work-hardens and stretches, and a stretched panel has excess material that has to go somewhere — which is how you get oil-canning, where the panel pops between two stable positions and will not sit still.
Shrinking stretched metal
When metal has been stretched — by collision, by overworking, or by welding heat — it needs shrinking rather than more hammering.
A shrinking disc is a smooth steel disc on an angle grinder that heats a small area through friction, which is then quenched, drawing the metal in. It is forgiving and it works well on gentle crowns.
Torch shrinking heats a small spot to a dull red, which the surrounding cool metal constrains so the hot spot swells upward, then quenching draws it in. Effective and easy to overdo.
A shrinking hammer with a textured face gathers metal mechanically.
The discipline in all of these is small amounts and frequent checking. Shrinking too far creates a low spot, and now you are stretching again.
Checking your work
You cannot see a quarter-millimetre wave by looking at a panel straight on. Three methods find what the eye misses.
Sight along the panel toward a light source. Crouch down and look across the surface at a shallow angle. Waves and low spots cast shadows that are obvious from this angle and invisible from any other.
Run your hand over it with a thin glove or a cloth. The hand is remarkably sensitive to changes in contour — considerably more sensitive than the eye — and a glove removes the friction that masks fine detail.
Use a flexible file, or a long board with coarse paper, on bare metal. It cuts the high spots and skips the low ones, which maps the surface immediately.
Filler, Used Correctly
What it is for
Filler levels minor surface irregularities on metal that is already the right shape. Measured in fractions of a millimetre, feathered to nothing at the edges, over clean, abraded, bare or epoxy-primed metal.
What it is not for
Building a contour. Filling a dent you could not be bothered to work out. Covering rust. Bridging a hole. Replacing metal.
Filler over corrosion is the most common bad repair in the hobby, and it fails predictably: the corrosion continues behind the filler, the oxide expands, and within a couple of years the repair telegraphs through the paint in a spidery pattern and bubbles at the edges.
The types
Polyester body filler is the general-purpose product — easy to shape, sands readily, and the standard for levelling work.
Reinforced fillers with glass or metal content are stronger and more waterproof but much harder to sand. Their place is over a welded repair for added strength, with a skim of regular filler on top for finishing. They are not a substitute for metal.
Glazing and finishing putty is a fine-grained product for pinholes and the last traces of texture. Modern two-part catalysed glaze is far better than old single-part air-dry putty, which shrinks and can cause solvent problems under paint.
Application that does not fail
Clean and abrade the substrate. Filler needs mechanical grip. A coarse abrasive scratch pattern is correct — glass-smooth metal gives nothing to hold.
Mix the correct ratio of hardener. Too little and it never cures properly and stays soft underneath. Too much and it cures too fast, becomes brittle, and can bleed colour through the paint.
Press the first coat in hard with a firm spreader to wet the surface and eliminate air. Air pockets become pinholes, and pinholes are discovered during paint, which is the worst possible moment.
Apply thin coats, built up rather than one thick application. Thick filler cures unevenly and shrinks as it continues to cure, which is why a repair can look perfect and then show a low spot weeks later.
Shape it while it is green — that brief window when it has set but is still soft enough to cut quickly with a cheese-grater file. Removing bulk here saves an enormous amount of sanding.
Feather the edges to nothing. A filler edge that steps rather than feathers will show as a visible line through the paint.
Never apply filler over paint, primer surfacer, or body seam sealer unless the product is specifically rated for it. Bare metal or epoxy primer is the correct substrate.
Block Sanding
This is the technique that produces flat panels and it is almost entirely about tool choice and direction.
Use long, rigid blocks
A long board follows the overall plane of the panel and cuts the high spots. A short block, and especially your hand, follows the existing contour — including the waves — so it reproduces them faithfully at a lower level.
This is the single most important mechanical fact in bodywork. Palm sanders and short blocks hide waves; long boards find them. The longer the block, the flatter the result, which is why professional work uses boards that look absurdly long to anyone who has not done it.
Use a rigid block on flat and gently crowned areas. Use flexible pads only where the panel genuinely curves in two directions at once, and keep them as rigid as the curve allows.
Cross-hatch
Sand in one direction at roughly forty-five degrees to the panel's long axis, then change to the opposite diagonal for the next pass. This prevents you cutting grooves that follow your own strokes and ensures the surface is being evaluated from two directions.
Never sand back and forth in one line along a panel. That produces channels.
Guide coats
The most useful cheap tool in bodywork. A light dusting of contrasting dry powder or aerosol over the surface before sanding. As you block, the guide coat is removed from the high spots and remains in the low ones.
When the guide coat is gone everywhere, the surface is flat. While any remains, you have a low spot — exactly located, visible, unambiguous.
Guide coat between every stage. It converts an invisible problem into a visible one, which is the whole difficulty in bodywork.
Grit progression
Work coarse to fine and never skip more than one step, because a coarser scratch has to be removed by the next grade and a jump leaves scratches that show through paint.
Roughly: shape filler with a coarse grade, level with a medium grade on a long board, refine, then finish to the grade your primer manufacturer specifies. Each stage removes the previous stage's scratches, and the final grade determines whether the sealer and base coat lay down without telegraphing sanding marks.
Primer surfacer is part of the process
High-build primer surfacer exists to be sanded. It is a thick, sandable layer applied over the epoxy-primed and filled surface, and blocking it flat is where the final flatness is achieved.
Expect to apply it, block it, find low spots, spot-fill or re-apply, and block again. Two or three cycles is normal on a panel being taken to a high standard. Anyone who sprays primer once, sands it lightly and calls it ready is producing a car that will show waves in sunlight.
Panel Gaps
Set them in bare metal, before anything else
Hang the doors, hood, trunk lid and fenders on the actual shell and adjust them to correct, even gaps — before filler, before primer, before paint.
This feels like wasted effort because you are about to take them all off again. It is not. Setting gaps after paint means either accepting bad gaps permanently or cutting and refinishing completed work, and blending new paint into old is difficult and often visible.
What to adjust
Hinge position, striker and latch position, shims behind bolt-on fenders, and on many designs the mounting points themselves have slotted holes with meaningful adjustment range.
Check that each gap is even along its length, that it matches side to side, that body lines align across the gap, and that the panel sits flush with its neighbours rather than proud or sunken.
When gaps will not come right
If a gap cannot be adjusted into alignment no matter what you do, the structure is wrong. Either the shell has been in a significant collision, or — more commonly on a restoration — the structure that was holding the openings square has been cut out for rust repair and the shell has relaxed.
This is why bracing matters. On a car with significant rot in rockers, floor or pillars, the door openings should be braced with tack-welded bars before the structural metal is cut out. Preventing the relaxation costs an hour; correcting it afterwards means pulling the body back to square, which is a different job entirely.
Measure diagonals across the shell and across each opening. They should match. If they do not, nothing else about the panels will come right.
Working on Different Metals
Steel is the easiest to work: it moves predictably under a hammer, it welds readily, and it tolerates heat reasonably.
Aluminium is a different craft. It work-hardens much faster, it has no colour change to indicate heat so it is easy to melt, it requires different filler and different primers to prevent galvanic corrosion, and it needs separate tools — using steel bodywork tools on aluminium embeds steel particles that then corrode. Panels crack rather than stretch when overworked, and welding distorts them far more than steel.
Galvanised and coated steel needs the coating removed locally for welding, and that removal creates a corrosion site that must be primed. Weld-through primer on mating surfaces is essential.
Fibreglass and composite panels need entirely different repair methods — resin and cloth rather than metal work — and different fillers. They also move differently with temperature than the steel around them, which affects gap setting.
Fitting Reproduction Panels
A practical reality of restoration work: reproduction panels vary enormously in quality, and even good ones rarely fit without adjustment.
Expect to rework them. Stamping dies for reproduction panels are not the factory dies, and the steel may be a different gauge. Common problems are body lines that do not align with the adjacent original panel, a crown that is slightly too flat or too full, flanges at the wrong angle, and overall dimensions that are close but not exact.
Fit it before you cut anything away. Hold the new panel against the car and compare the body line position, the crown and the flange angles against the original. Finding out it is wrong while the original is still in place means you still have a pattern.
Trial-fit repeatedly. Clamp, mark, trim, clamp again. A panel that has to be forced into position carries stress and will never sit right — and worse, the stress will show as a distortion in the paint months later as it relaxes.
Check the body line continuity at the joint, because that is what the eye follows. A body line that steps or kinks where the repair panel meets the original is the single most visible failure in a panel replacement, and it is far more noticeable than a slightly imperfect surface.
Compare the flange depth where the panel meets a door or opening, because this sets the gap and the flush fit.
Consider a partial panel. For many repairs, cutting a section from a full reproduction panel and butt-welding it into the original is better than replacing the whole panel — it preserves the original body line position and reduces the number of joints in visible areas. The joint should be placed somewhere the eye does not naturally travel, and never along a body line.
Galvanised reproduction panels need the coating removed locally at weld areas, and the removal creates a bare site that must be primed with weld-through primer on the mating faces and epoxy afterwards.
The Mistakes That Show Up in the Paint
Too much filler. Cracks, shrinks and telegraphs.
Filler over rust or paint. Fails from behind.
Short blocks and palm sanders. Produce a surface that follows its own waves.
Skipping guide coats. Means working blind.
Jumping grits. Leaves scratches that appear under clear coat.
Feather edges that step rather than taper. Show as visible lines.
Sanding through to bare metal and priming over it with only surfacer. Primer surfacer is not a corrosion barrier. Any bare metal exposed during blocking needs epoxy primer before surfacer goes back on, or you have built a rust site under the paint.
Pinholes left unfilled. Appear as tiny craters in the clear, and they are very difficult to fix afterwards.
Grinding welds too thin. Creates a low spot that needs filling and a fatigue point.
Working in poor light. The most underrated cause of bad bodywork. You cannot find what you cannot see. Strong, movable, raking light from multiple angles is as important as any tool, and a well-lit home shop will outperform a badly lit professional one.
Rushing the last stage. The final blocking and the final primer cycle are where the difference between good and excellent lives, and they are the stages that feel most like they could be skipped.
Tools That Matter, and What They Do
Bodywork rewards the right tool more than almost any other phase, and most of the useful ones are inexpensive.
Long sanding boards, in several lengths. The most important tool in the phase. Anything shorter than a hand's width is for detail only.
A flexible file, sometimes called a vixen file or a body file, with coarse curved teeth on a flexible holder. It cuts bare metal and green filler fast and it maps the surface as it goes, because it only touches high spots.
A cheese-grater file for shaping filler in the green stage. Removing bulk here rather than with abrasive paper saves hours.
A good hammer and dolly set. A bumping hammer, a pick hammer, and a selection of dollies in different radii. Buying the cheapest set means hammer faces that are not properly dressed, which leaves marks you then have to remove.
A shrinking disc for pulling stretched metal back in. One of the highest-value tools for anyone repairing collision damage or working after welding.
A stud welder and slide hammer for pulling dents out from the outside where there is no access behind the panel. This is how you avoid drilling holes in a panel to pull a dent, which is the old method and leaves corrosion sites.
A shrinker-stretcher for forming flanges and curved panel edges. Essential if you are fabricating any patch with a flange.
A bead roller for reproducing swage lines and stiffening ribs. Not essential for most repairs, transformative when you need one.
Guide coat, as powder or aerosol. Costs almost nothing and is the difference between working with information and working blind.
Strong, movable, raking light. The most underrated item on the list. A single overhead fluorescent tube is close to useless — you need light you can position to rake across a panel at a shallow angle, from several directions, because that is the only way surface irregularities become visible.
A paint depth gauge is worth having by the end, when you are polishing, and useful earlier for finding filler on a car with unknown history.
How Much Is Enough
A legitimate question, because the standards differ enormously and so does the labour.
For a driver-quality car, the target is a surface that looks excellent from a few feet and in normal light, with correct gaps and no visible repairs. This is achievable in a reasonable number of hours and it is what most people actually want.
For a show car, the target is a surface that is genuinely flat under direct raking light from any angle, with reflections that run uninterrupted across the whole panel. The additional work is almost entirely in repeated primer and blocking cycles, and it is a large multiple of the driver-quality hours.
Decide which you are building before you start, and be honest. The temptation to chase show-car flatness on a car you intend to drive is how projects run out of budget in the primer stage — and a well-finished driver that gets used is worth more than a perfect panel on a shell in storage.
Straight Answers
Common Questions
How much filler is acceptable?
As a skim to level minor imperfections after the metal is already straight — generally measured in fractions of a millimetre, not millimetres. Filler is a levelling compound, not a shaping material. If you are building shape with it, the metal underneath needs more work with a hammer, dolly and shrinking disc first.
Why does my paint show waves under certain light?
Because the substrate was never actually flat. Waves, low spots and sanding scratches are invisible in a dim shop and obvious under sunlight or showroom lighting. Guide coats and long boards find them; palm sanders and short blocks follow the existing contour and hide them until the clear goes on.
When should panel gaps be set?
In bare metal, before any filler or primer, with the doors, hood and trunk hung and adjusted on the actual shell. Setting gaps after paint means either accepting bad gaps or cutting into finished work. Hang the panels, set the gaps, then take them off again for finishing.
Do I need a professional booth for good results?
For the final colour and clear, a clean controlled environment makes an enormous difference to the amount of finishing work afterwards. For everything before that — metal work, filler, primer and block sanding, which is the great majority of the labour — a well-lit, organised home shop is entirely capable.