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I won't go into the actual low-level chemistry (especially because it's totally different between polyesters/vinylesters and epoxies) but essentially tooling gel-coats are very much the same family as regular gelcoats however the are engineered to prioritise certain properties (hardness, polishability and dimensional stability). This might not sound very different to a regular gel coats and in fact they're not but they will traditionally all include fillers (to improve their dimensional stability) whereas generally a gel coat that's designed for parts will not have fillers (because its appearance, either clarity or ability to accept a pigment) is very important. The hardness of a tooling gelcoat can also be higher because they're designed to be the surface for a rigid tool; this means that they don't need to be 'flexible' in the way that a gel coat designed for parts would need to be to prevent it from cracking if the part flexes.
The bottom line, in practical terms, is that tooling gel coats and regular gel coats are chemically similar but tooling gel coats can take advantage of the fact that they won't be flexed and don't need to be clear or easy to pigment. These distinctions mean they can be much better gel coats as a mould surface (harder and filled) in a way that a regular gel coat cannot.
I hope this helps,
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