Concrete mold material? Bondo?

[mschrank]

I'm in the early planning stages of a project for next spring.

I need to make several cast concrete slabs about 51" x 11" x 3.5"

These will be poured and cured, then removed from the forms and placed.

I would like to add some detail along one long edge and one short edge on each slab. Basically, I'd like to make the edges look like rough cut stone.

I'm envisioning using some existing rough-cut stone I have on hand to make an impression in something that is initially soft but will then harden. This would then be placed in the wood form.

Would Bondo work for this? I'm thinking the concrete would adhere to it...? Could I coat it with oil/release agent? I'd like to be able to reuse this "mold" a couple of times.

Any other ideas? I don't really want to get into the pro-style rubber mold materials...but maybe that's the way to go?

[scorrpio]

Whatever you use to create the texture (joint compound seems like a cheap option), just wrap it in thin plastic once it dries. I don't think concrete would stick to plastic...

[Cheeky]

I made my own concrete countertops which came out fantastic.

Fu Tung Cheng's books helped out tremendously, and I'm positive that his casting/mold techniques will help you out immensely. Also, he has links for color, texture and techiniques for finishing.

They have the books @ Borders, Barnes and Noble, so if you just want to peruse them that will be a good source.

I don't know what you have on deck, but this should help, if not, sorry:

http://www.concreteexchange.com/catalog/1/

[crokett]

Wet sand. Build your form then add sand and shape to the profile you want. Then do your pour. We used sand as a mold to pour some stepping stones for my mom a few years ago.

[linear]

I'd use styrofoam insulation sheets.

Another valuable web resource:
http://www.the-artistic-garden.com/hypertufa.html

[cabinetman]

You could make a wood box that gets screwed together, including the sides to the bottom. For the edges that you want free form, you could form them with chicken wire, or screening, then use fiberglass cloth to get the shape you want then brush on the resin. When dry just disassemble.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[mschrank]

Thanks for all the great suggestions

I'll let y'all know what I end up doing and post some pics if it comes out OK. This will be a winter project in the garage after the kitchen remodel, so may be awhile.

Although its a ways off, I asked for help because this is keeping me up at night...

[Daryl]

Bondo can be pretty brittle and I think the concrete would bond to it. My suggestion is to use silicone caulk for the mold.

[eddy merckx]

The way that exposed aggregate concrete is made is to coat the top with molasses. It keeps the concrete from curing. I wonder if a thin coat would make a good release agent against your bondo mold.