Could you tell me what points must be watched when formulating a repair filler based on an unsaturated polyester resin ?
The most important condition for formulating a UP repair filler which can be easily sanded, is fast curing and flexible is to use unsaturated polyester resins which are pre-accelerated with special tertiary amines. To ensure perfect adhesion to the metal surface, no paraffins should be used in the formulation.
It is best to use a mixture of highly and medium reactive polyester resins which cured by adding 1% of a 50% benzoyl peroxide paste. Such a repair compound, filled with talc, gypsum and lithopone can be sanded already after 20 minutes. This time interval can be tripled or quadrupled by reducing the reactivity of the formulation
Other kinds of polyester resin can also be used e.g. highly flexible, pre-accelerated resins. These are filled with microtalc, whiting, powdered mica and powdered slate and hardened by adding 1 - 3% of a 50% benzoyl peroxide paste. The pot life and cure time of such a formulation can be increased by adding 0,5% of a tertiary butyl pyrocatechol solution ( 1% ) in toluene.