They cure by completely different chemistry
Polyester putty cures by free-radical crosslinking. Styrene copolymerises across the unsaturation in the polyester chain, started by a peroxide at a low catalytic percentage. The reaction involves volatile styrene and produces meaningful cure shrinkage.
Epoxy putty cures by polyaddition. The epoxide groups and the amine hardener react stoichiometrically, every molecule of hardener taking part, with no volatile loss and correspondingly lower shrinkage.
That difference explains almost everything else, including the ratio discipline. In polyester the peroxide is catalytic, so more of it does not give a harder cure, it gives staining and gassing. In epoxy the hardener is a co-reactant, so getting the ratio wrong genuinely leaves unreacted material and a permanently inferior cure. The same instruction, mix accurately, is right for opposite reasons.
Why polyester owns the body-filler market
Speed is the decisive factor. A body filler goes from mixed paste to sandable in the order of fifteen to twenty minutes at workshop temperature, and a competent technician can shape it in the green stage with a body file within minutes. Epoxy is much slower.
Sandability is the second. Talc-filled polyester cuts cleanly and finishes well, and the whole repair process is built around that behaviour.
Buildable volume is the third. Filler is applied in real thickness to restore panel shape, and heavy mineral loading makes that affordable. Epoxy at the same volume would cost several times more, is more viscous and is harder to shape.
Cost is the fourth, and in a trade where materials are a per-repair cost it matters.
Where epoxy genuinely wins
Adhesion is the clearest case, particularly on difficult substrates and on plastics. Epoxy is also the basis of the corrosion-protection primer that many procedures call for on bare steel, which is a different job from filling but sits adjacent to it in the process.
Low shrinkage matters wherever dimensional accuracy is the point rather than cosmetic restoration: pattern work, tooling repairs and precision fits.
Flexibility and impact resistance matter on plastic components. It is worth noting that some products sold as flexible fillers for bumpers are epoxy rather than polyester, which is a genuine category boundary rather than a branding difference.
Moisture resistance is the last. Epoxy's polyaddition network and better adhesion give it an advantage where water exposure is sustained.
Where each one fails
Polyester fails over acid. Self-etch and acid primers, and acidic preparation wipes, inhibit polyester cure, and this is a hard prohibition rather than a preference. It also fails at excessive thickness, where trapped exotherm gasses the film and leaves an undercured core, and it fails cosmetically through shrinkage and scratch telegraphing when applied thick or sanded early.
Epoxy fails on speed and cost in a production repair environment, and it fails when the ratio is wrong, because unlike polyester there is no catalytic forgiveness. Amine blush and surface contamination between coats are its own class of problem.
Neither belongs over rust. Filler is not a barrier coating, rust is not a bondable substrate, and the oxide keeps growing beneath whatever is put on top of it. There is a genuine industry argument about whether rust appearing under repairs originates from the filler or from environment and time, but both readings end at the same instruction: remove the rust, protect the steel, then fill.
What this means for a putty manufacturer
If you are formulating a product to compete in the body-filler market, polyester is the chemistry that market is built around, and the competitive axes are working time, sanding window, pinhole freedom, feather-edge behaviour and adhesion across the substrate list.
If you are asked for a filler for flexible plastics, dimensional work or sustained water exposure, that is a different product and often a different chemistry, and it is more honest to say so than to stretch a polyester formulation to cover it.
Samrat Poly Resins supplies Polyester Putty Resin as a putty base, and separately manufactures epoxy resin systems. The two answer different questions rather than competing for the same one.