Both routes are real, and the absolutes are wrong
You will see it stated that polyester putty must use benzoyl peroxide with an amine, and you will see it stated that body-filler hardener contains MEKP. Neither absolute survives contact with the literature.
What is accurate is that benzoyl peroxide with a tertiary amine dominates hand-applied automotive body filler, while ketone peroxides with cobalt appear in sprayable polyester putties and in some marine and general fairing putties. Two products in the same category can legitimately use different peroxides.
The practical consequence for anyone buying or formulating is that the cure system is a property of the specific product, and hardeners are not interchangeable between them.
The benzoyl peroxide and amine route
This is the classic body-filler system and the source of the familiar red, blue or cream hardener paste. The hardener is benzoyl peroxide dispersed in a plasticiser carrier with a dye, and the resin side carries a tertiary amine such as dimethyl-p-toluidine, often already built in by the resin maker.
The chemistry is an amine and peroxide redox pair. The amine attacks the peroxide to generate radicals at ambient temperature, so no heat and no cobalt are required.
The technical reason it dominates filled putty is specific and worth knowing. Peroxide producers describe this system as showing a very fast cure that is hardly influenced by humidity and fillers, and as giving a relatively good cure even at low temperatures. Insensitivity to filler load and humidity is precisely what a paste carrying half its mass as mineral, applied in a humid workshop, needs.
Its documented disadvantage is yellowing and poor light resistance in the cured film. For body filler that is irrelevant, because the putty is sanded and buried under primer and topcoat.
The ketone peroxide and cobalt route
Cyclohexanone peroxide and MEKP with a cobalt accelerator is the system used across laminating and casting polyester, and it is genuinely used in putty as well. Peroxide producers describe the cyclohexanone peroxide and cobalt system as particularly suitable for lacquers and spray putties, and note that it makes light-resistant parts possible, in contrast to the benzoyl peroxide and amine system.
That is the trade-off in a sentence. Where the finished surface will be visible or where yellowing matters, the ketone peroxide route has an advantage. Where the product is a heavily filled paste that will be sanded and painted over, the benzoyl peroxide route's insensitivity to filler and humidity wins.
The market bears this out. Two marine polyester fairing compounds from different suppliers use different peroxides: one is catalysed with MEKP, the other requires a benzoyl peroxide hardener at a stated weight ratio. Same product category, two different systems.
Why the hardener is coloured
The dye in a cream hardener is a mix indicator. Because the correct dose is a low single-digit percentage of a stiff paste, and because nobody in a body shop weighs it, the colour is how the user confirms the peroxide has been folded uniformly through the mass. Streaks mean unmixed putty, and unmixed putty means soft patches.
This is also why finished-putty makers insist on their own matched hardener. The dose is calibrated to the peroxide concentration in that specific paste, to the amine level in that specific resin and to the target working time. Substituting a hardener from another product changes all three at once.
Why both too much and too little hardener cause staining
This is the most useful thing a body shop can learn about polyester cure, and it is counter-intuitive because the error is symmetrical.
Too much peroxide leaves free oxidant that can migrate upward and attack basecoat and clearcoat pigment, and the additional exotherm gasses the film and produces pinholes. Too little leaves an incompletely cured putty with unreacted species that migrate up through primer and base to discolour the clear. Both ends produce staining and bleed-through.
Manufacturers say this explicitly. One major filler data sheet carries the instruction not to under-catalyse in capital letters, while another warns against exceeding the recommended hardener level to avoid bleeding and spot marking. The correct ratio is the only safe answer, and the word hardener is itself part of the problem, as the next section explains.
Hardener, catalyst and accelerator are three different things
Strictly, the peroxide is an initiator. It is used catalytically at a low percentage and is not consumed stoichiometrically. The accelerator or promoter is the tertiary amine, in benzoyl peroxide systems, or the cobalt salt, in ketone peroxide systems, and its job is to make the peroxide decompose at room temperature.
Hardener is a colloquialism borrowed from epoxy, where the hardener genuinely is a stoichiometric co-reactant and where using more of it does change the ratio. That borrowed word is the direct source of the belief that more hardener gives a harder filler. In a polyester system it gives staining and pinholes instead.
Getting the vocabulary right is not pedantry here. It prevents a specific, common and expensive mistake.
The safety line that matters most
Benzoyl peroxide and MEKP are different peroxides with different accelerators, different dose regimes and different hazards, and they must never be casually interchanged. Benzoyl peroxide is classified as an organic peroxide that may cause fire or explosion on heating, as an eye irritant and as a skin sensitiser. MEKP is corrosive and a serious eye hazard.
Both have real storage constraints. Recommended maximum storage temperatures around twenty-five degrees Celsius, with self-accelerating decomposition temperatures not far above that, are a genuine issue in an Indian warehouse rather than a formality.
And the absolute rule: never bring a peroxide and an accelerator together directly. Accelerator goes into the resin first, peroxide last, each handled separately. This applies with particular force when a resin arrives pre-accelerated, because the amine is already in the drum.