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For short run stuff, or an intermediate mold I wouldn't form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement.

For something really durable, I'd probably go with a resin impression coat, and lay up some fiberglass of some sort under that. Maybe just a healthy layer of Bondo-Glass. (Which is not like regular Bondo. It's not easy to sand, and it's much stronger, because it's filled with glass fibers instead of little microbubbles.)

If I still wanted to be able to sand and modify it easily, at the expense of making it somewhat less durable, I might start with a layer of regular Bondo. Then, deeper than I'd ever want to sand away, a healthy layer of Bondo-Glass for overall strength.

BTW, don't trust me on this. This resin/glass stuff is all still on my to-do list, not something I've actually done. (Today I'm experimenting with hot wire cutters for the foam part... I'll get there eventually.) I have done plaster waste molds and reinforced water putty casts, but not fiberglass.

Paul

<blockquote data-quote="drcrash" data-source="post: 104182" data-attributes="member: 1623">For short run stuff, or an intermediate mold I wouldn&#039;t form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement.For something really durable, I&#039;d probably go with a resin impression coat, and lay up some fiberglass of some sort under that.&nbsp; Maybe just a healthy layer of Bondo-Glass.&nbsp; (Which is not like regular Bondo.&nbsp; It&#039;s not easy to sand, and it&#039;s much stronger, because it&#039;s filled with glass fibers instead of little microbubbles.)If I still wanted to be able to sand and modify it easily, at the expense of making it somewhat less durable, I might start with a layer of regular Bondo.&nbsp; Then, deeper than I&#039;d ever want to sand away, a healthy layer of Bondo-Glass for overall strength.BTW, don&#039;t trust me on this.&nbsp; This resin/glass stuff is all still on my to-do list, not something I&#039;ve actually done.&nbsp; (Today I&#039;m experimenting with hot wire cutters for the foam part...&nbsp; I&#039;ll get there eventually.)&nbsp; I have done plaster waste molds and reinforced water putty casts, but not fiberglass.Paul</blockquote>

[QUOTE="drcrash, post: 104182, member: 1623"] For short run stuff, or an intermediate mold I wouldn't form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement. For something really durable, I'd probably go with a resin impression coat, and lay up some fiberglass of some sort under that. Maybe just a healthy layer of Bondo-Glass. (Which is not like regular Bondo. It's not easy to sand, and it's much stronger, because it's filled with glass fibers instead of little microbubbles.) If I still wanted to be able to sand and modify it easily, at the expense of making it somewhat less durable, I might start with a layer of regular Bondo. Then, deeper than I'd ever want to sand away, a healthy layer of Bondo-Glass for overall strength. BTW, don't trust me on this. This resin/glass stuff is all still on my to-do list, not something I've actually done. (Today I'm experimenting with hot wire cutters for the foam part... I'll get there eventually.) I have done plaster waste molds and reinforced water putty casts, but not fiberglass. Paul [/QUOTE]

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