Gummy on top, hard underneath
This is the most common complaint and usually the least serious. Free-radical polyester cure is inhibited by atmospheric oxygen at the exposed surface, so the film can be properly cured through its thickness while the top few microns remain tacky.
The conventional countermeasure is a small quantity of paraffin wax in the formulation that is incompatible with the resin, migrates to the surface during cure and forms a physical oxygen barrier. Putty resins are often sold on this air-drying or tack-free character, and it is a formulation property rather than a workshop error.
Distinguish it carefully from the genuine fault below, because the remedies are different and adding more hardener to an air-inhibited surface makes things worse rather than better.
Soft all the way through
This is genuine undercure and it has a short list of causes. Under-catalysing is first. Expired or non-matching hardener is second, and it is worth remembering that peroxides lose activity in storage and have real temperature limits.
Cold is third, and it includes the case where the product is warm but the panel is not, since the substrate pulls heat out of a thin film. Poor mixing is fourth: folding rather than stirring is the correct technique, and streaks of unmixed colour are the visible warning.
Fifth, and frequently overlooked, is substrate chemistry. Acid inhibits polyester cure, so applying over a self-etch or acid primer, or over a surface still carrying an acidic preparation wipe, will leave the bond line soft no matter how well the mix was done.
Pinholes
Pinholes come from two directions, and separating them decides whose problem it is.
In the workshop, they come from air whipped in by stirring instead of folding, from over-catalysing where the extra exotherm gasses the film, from applying too thick a layer so exotherm is trapped, and from sanding before the putty has properly cured so that voids open in a still-plastic matrix.
In the formulation, they come from over-filling. Above the point where there is enough resin to fill the voids between filler particles, air voids simply remain in the cured film. A putty that pinholes consistently across users, applied correctly, is telling you something about its filler loading rather than about the technicians.
Crushed hollow microspheres are a third and more specific formulation cause, since broken bubbles release trapped air into the paste.
Staining and bleed-through into the paint
This is the failure that surprises people, because both ends of the mixing error produce it.
Too much peroxide leaves free oxidant in the cured putty that can migrate upward and attack basecoat and clearcoat pigment. Too little leaves incompletely cured material whose unreacted species migrate up through primer and base to discolour the clear coat.
Manufacturers say both. One major data sheet instructs users in capitals not to under-catalyse; another warns against exceeding the recommended hardener level to avoid bleeding and spot marking. Since both errors give the same symptom, the correct ratio is the only remedy, and eyeballing a ribbon of hardener across a puddle degrades as the puddle gets bigger.
A separate and preventable cause is failing to seal the filler. Cured putty is porous and talc-bearing, and it will absorb and hold solvent. Unsealed filler produces staining, sand-scratch swelling and topcoat adhesion loss, which is why colour and clear should never see raw filler.
Edge mapping and scratch telegraphing
Edge mapping, sometimes called ringing, is the halo that appears around a repair after the topcoat goes on. It has three contributing causes and they compound.
Shrinkage is the first, worsened by thick application, over-catalysing and premature sanding. A poor feather edge is the second: if the filler does not taper cleanly to nothing, the transition will show. Working from the edges toward the centre is the technique that prevents it.
Coarse sand scratch is the third. A grit scratch appropriate for bare metal will not be filled by primer and will telegraph through the topcoat, which is exactly why a fine, low-shrink glazing putty exists as a step between the body filler and the primer.
Applying too thick, and why the limit is real
Every major finished-filler data sheet caps thickness, and the caps differ by product, so a universal number is not worth publishing. What is worth understanding is why the limit exists.
A thick fill traps its own exotherm. That gasses the film into pinholes, increases shrinkage and can leave the core undercured even while the surface is hard. The consistent instruction across manufacturers is to build in multiple thin layers rather than one heavy application.
The same logic explains why fibre-reinforced fillers, which cure very hard and are difficult to sand, are normally skimmed over with a talc-filled grade rather than finished directly.
Which of these are formulation problems
It is worth being straight about where responsibility sits, because a resin supplier is only accountable for part of this.
Consistent pinholing under correct application, poor feather-edge behaviour, a paste that sags on vertical panels, a surface that never goes tack-free, poor sandability and excessive shrinkage all point back toward the formulation, and through it to the resin, the filler package and the thixotrope.
Under-catalysing, poor mixing, application over acid primer or rust, excessive thickness and premature sanding are workshop causes. Substituting a non-matching hardener sits in between, since it is a user action that a manufacturer can only prevent by labelling clearly.
Mixing ratios, it is worth repeating, come from the finished putty maker's own instructions. A resin supplier can say what a resin is designed for; it cannot specify the hardener dose for someone else's finished product, because that depends on the peroxide concentration, the amine level, the filler package and the target working time.