Talc, and why it is almost always the base filler
Talc is the softest mineral on the Mohs scale and it is lamellar, cleaving easily along basal planes. Both properties matter. Softness means it does not blunt the abrasive, which is the direct reason talc-filled putties sand easily and finish well.
That advantage is conditional on purity. Hard accessory minerals, quartz above all, cause scratching and wear the paper rapidly. A talc specified only by particle size, with no statement about accessory minerals, is not fully specified for this use.
Producers cite three further reasons. Low and consistent oil absorption means less resin is needed for a given rheology, which is both a cost and a styrene-reduction lever. The platy shape gives a large parallel contact area at the substrate. And talc's hydrophobic surface, with platelets aligned in the film, creates a tortuous path that improves moisture resistance.
Bimodal sizing is real practice rather than theory: a fine fraction packs the interstices for surface smoothness while a coarse fraction carries bulk loading at low resin demand.
Calcium carbonate, and where it stops
Ground calcium carbonate is a cost and volume extender with low oil absorption. Precipitated grades are finer, higher in surface area and higher in oil absorption, and are usually surface treated.
The limit is hardness. Calcite is several times harder than talc on the Mohs scale, so carbonate is the more abrasive of the two and is normally used as a partial replacement rather than a base. Patent formulations commonly pair a small percentage of carbonate against a much larger talc fraction.
Carbonate is also acid sensitive, which is chemically uncontroversial and relevant wherever acid-etch primers or acidic preparation wipes are in the process. We have not found a primary source quantifying that effect inside a cured polyester putty, so it is better treated as a reason for caution than as a measured penalty.
Hollow microspheres and the lightweight grades
Hollow glass microspheres are what separates a lightweight filler from a standard one. They are non-porous, so unlike a porous extender they do not absorb resin, and their volumetric effect is dramatic: a kilogram of glass bubbles displaces roughly an order of magnitude more volume than a kilogram of carbonate. Density therefore falls sharply at a low weight fraction, which is why patent formulations use only a few per cent.
Their spherical shape also gives a ball-bearing effect and lower resin demand than platy talc, so total filler volume can rise without the viscosity climbing out of control.
The manufacturing risk is crush, and it is specific enough to be worth stating plainly. Microsphere producers advise avoiding high-shear processes, naming high-speed dissolvers, gear pumps and three-roll mills. Broken bubbles stop reducing density, release trapped air and raise resin demand, so a formulator who adds microspheres without changing the mixing regime can end up with a heavier, more porous product than before.
Fumed silica and how non-sag behaviour actually works
Fumed silica particles carry surface silanol groups that hydrogen bond to one another, building a temporary three-dimensional network through the liquid. Macroscopically that reads as thickening. Under mechanical load the network breaks down and viscosity falls; at rest the particles re-associate and viscosity rebounds.
That is exactly the putty requirement: yield instantly under the spreader, then rebuild before gel so the paste does not slump off a vertical panel.
Two consequences from the additive literature are worth knowing. The thixotropic effect depends on system polarity and actually rises as styrene content rises, because a less polar resin wets the silanol groups less and lets them bond to each other more. So cutting styrene for regulatory reasons reduces the efficiency of the thixotrope, which is a genuine formulation tension rather than a free choice. And the temperature during dispersion affects storage stability, with higher dispersion temperatures increasing the tendency to sediment.
It is also worth noting that some microcrystalline talc grades act as thixotropes in their own right and can partly replace fumed silica in polyester systems.
The one concept that explains porosity, pinholes and crumbling edges
Critical pigment volume concentration is the idea that ties the failures together. Above it, there is simply not enough resin to fill the voids between filler particles, so air voids remain in the cured film and properties fall away.
That single mechanism explains why an over-filled putty is porous and pinholes on sanding, and why a resin-starved feather edge crumbles rather than thinning away to nothing. It is a first-principles explanation rather than a vendor claim, and it is more useful than any rule of thumb about loading percentages.
The opposite error has its own signature. A resin-rich putty smears under the abrasive and loads the paper instead of cutting cleanly.
How loading drives every other property
Shrinkage falls as filler volume rises, because all cure shrinkage comes from the crosslinking of styrene with the polyester and the mineral takes no part in it. Sag resistance rises with loading and with platy or microcrystalline talc.
Density is essentially a volume-weighted mix, and only microspheres move it meaningfully. Resin demand is set by oil absorption and particle shape, with talc commonly described as topping out around half the mass without a wetting aid and substantially higher with one.
Sandability is a function of crosslink completeness, backbone rigidity and the filler package together. Flexible polyester formulations are documented as sanding badly, which is why a flexible putty for plastic substrates is a genuinely different product rather than a softer version of the same one.
The other fillers, briefly
Wollastonite is acicular and reinforcing, adding crack resistance over welds at the cost of sandability, since it is considerably harder than talc. Mica is a high-aspect platelet that raises modulus and moisture and heat resistance.
Barytes is inert and very dense, used for damping in heavy stoppers and entirely wrong for a lightweight product. Aluminium powder replaces talc in metal-filled grades and is associated with increased strength, durability and water resistance, with some grades drillable and tappable; these characteristically use a liquid hardener rather than a cream.
Short or milled glass fibre adds strength and water resistance over weld seams and moulded composite panels, at a real cost in sandability, which is why fibre-reinforced fillers are normally skimmed over with a talc-filled grade.