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Thread: My '55 Belair 2 Door Hardtop Build (sort of

  1. #21
    Registered Member BamaNomad's Avatar
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    Tabasco, Yes what you said about Epoxy primer is correct (ie. you can do bodywork/smooothing with filler, etc) over it.

    ALL Epoxy paints that I've seen or used are two part (2-K if you prefer that term), but I like calling two part paints as two part paints because it's more clear what one means, meaning that you add an activator, catalyst, or 'hardener' which speeds up drying and makes the paint more durable.

    Urethane paints are two part paints but are not epoxy paints; Urethane paints use an 'activator' (or hardener if you will).

    One can even add an urethane activator/hardener to 'straight enamel' paints which will also make that paint more durable.

  2. #22
    Registered Member chevynut's Avatar
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    Quote Originally Posted by BamaNomad View Post
    meaning that you add an activator, catalyst, or 'hardener' which speeds up drying and makes the paint more durable.
    The hardeners, catalysts, activators, etc. used for 2K paints/primers do not speed drying. They cause or enhance a chemical reaction with the paint that causes the molecules to cross-link making it more durable. In fact, to be a "2K paint" there must be a chemical reaction, not just changing the paint in some way like adding thinner, etc. The urethane basecoat in BC/CC is typically not 2K but you can add a hardener if you want to. The clearcoat is 2K urethane.

    Urethane paints are two part paints but are not epoxy paints; Urethane paints use an 'activator' (or hardener if you will).
    Urethane is a different polymer than epoxy and requires a different activator. What happens with epoxies is the same thing that happens with urethanes, but different chemicals are used to cause the molecular cross-linking.

    It's similar to plastics. Thermoset plastics harden by chemically cross-linking the molecules and the material can't be melted down and re-used. A thermoplastic can be ground up and re-used since there is no chemical reaction during manufacture of the part.
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  3. #23
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    If you add catalyst to the old school acrylic enamel paint, it actually slows drying by quite a lot. And that crap didn't dry very fast to start with.

    Fortunately that's not an option any more if you want an OEM color. I guess you could add it to whatever colors Rustoleum or similar products come in.

    With 2k paint or primer, the catalyst is required if you ever want it to dry. You don't just shoot it without catalyst.

    Modern urethane with catalyst dries to the touch almost as quickly as old school lacquer.

    The catalyst for epoxy primer is not heavily laden with isocyanates (like the urethane paint) - but that doesn't mean you should spray it without proper PPE protection/procedures.

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