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need help with dome render cracking - Forno Bravo Forum: The Wood-Fired Oven Community

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Forno Bravo Forum Community,

You will notice a new forum at the top of the main page called, "Ask Me Anything". This forum will be used for live one hour "Ask Me Anything" (AMA) sessions hosted by people who are knowledgeable in different areas pertaining to wood fired ovens. How it works:
- Each AMA will have a "sticky" thread where the community can post questions they would like answered during the live session. This will allow everyone to participate even if you can't be online for the live session. These questions will not be answered by the host until the live AMA; if you need an answer quickly, you should post it in the appropriate Forum area for the community to respond.
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To kick off our AMA feature, we have invited author, chef and master bread maker and host of Pizza Quest, Peter Reinhart, to be our first host! Peter will be in the Forum on Monday, February 15th, from 7:00 - 8:00 pm EST. If you are unable to be online during the live session, you can post your questions in the sticky post. Peter will answer those questions during the live session on February 15th. You can view Peter's answers to your questions as well as what happened during the live session in the session thread.

We hope you enjoy this new feature! Please let us know if there is a topic that you'd like to have as an AMA and we'll look for a host!

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need help with dome render cracking

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  • need help with dome render cracking

    Hi all i have put on 3 coats of render and have just made beer can chicken
    all went great,exept the cracks running over my dome about 8 in total
    about 3mm wide,i have a coffee cake in there now,as the dome is cooling i can see some of them closing i want to move it out of the shed soon,
    any idea how i can water proof the dome or should i bight the bullet and build a roof over it?.The render{stucco}is about 15/20mm thick amd i fired it with the first coat on hoping to finish the cracking process.
    thanks for any ideas.ps i used comercial acrylic render from bunnings warehouse.
    Peter
    Last edited by toyota08; 10-26-2008, 01:35 AM.

  • #2
    Re: need help with dome render cracking

    I'm doing my firing prior to the render coat, so if there are any cracks in the vermicrete layer, the render will fill them. I'm half way through my firing process and there's only a couple of very fine hairline cracks where the vermicrete meets the refractory castable at the door arch. The render coat will easily fill these. No other cracks anywhere. It's been two weeks since I cast the dome and one week since the vermicrete layer so it's had some time to cure already.
    I'm going to use an external weatherproof acrylic/sand render called "Multitex"(they're in Preston).

    What insulation have you used and how long have you let your oven sit before firing? What was your firing procedure?
    Last edited by Balthazar; 10-26-2008, 04:21 AM.

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    • #3
      Re: need help with dome render cracking

      Hi mate i hade the oven sit a couple of weeks before firing at that time it had no render
      and for insulation it has 1.5"vermiculite 7-1 then 1/2" ceramic blanket chicken wire then 4" approx vermiculite 7-1.i then slowly fired it over 2 days left it a week did the same thing 4 weekends in a row.I then did the first render layer fired twice to get the cracks then added 2 more colored layers of rendar.I may just grind the cracks larger and cover them with mastic to keep the rain out.
      cheers Peter

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      • #4
        Re: need help with dome render cracking

        I think you need mesh in the final render shell. I tried using fibre instead of mesh to save time but it didn't work- cracked anyway so I rendered over it with another layer 15mm of render but this time with mesh. No cracks. If using a cement based render you should use lime as well because lime has some elasticity. The brew is 4:1:1 sand /lime/cement. If using an acrylic make sure you use one that is elastic or it will crack too.
        Kindled with zeal and fired with passion.

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        • #5
          Re: need help with dome render cracking

          Originally posted by david s View Post
          I think you need mesh in the final render shell. I tried using fibre instead of mesh to save time but it didn't work- cracked anyway so I rendered over it with another layer 15mm of render but this time with mesh. No cracks. If using a cement based render you should use lime as well because lime has some elasticity. The brew is 4:1:1 sand /lime/cement. If using an acrylic make sure you use one that is elastic or it will crack too.
          As I mentioned in my previous post, I'm using an acrylic(felixible) render called Multitex: Multitrowel
          Then I'm going to go over that with Wattyl Solaguard Stone Finish or Porter's Limewash which will fill any fine cracks....well that's the plan at least

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          • #6
            Re: need help with dome render cracking

            Thanks guys i was looking at the Bondall site they have a fine plastic mesh that you lay under thier pondtite flexible membrane i may look into it,i may also cut a expansion joint around the chimney area as some cracks seem to come from this area.
            cheers peter

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            • #7
              Re: need help with dome render cracking

              If using a cement render you should keep it damp for a week to allow it to achieve max strength. I find that wrapping it in cling wrap works really well. But then allow another week to let it dry out before firing again.
              Kindled with zeal and fired with passion.

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              • #8
                Re: need help with dome render cracking

                Also, adding some plasticiser and/or lime into the mortar will make it a bit more flexible. Are you rendering with sharp sand or soft sand?
                Matt S, Cambs, UK
                42" Pompeii

                Pizza oven pictures - WIP!

                Pizza oven costs (so far!)

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                • #9
                  Re: need help with dome render cracking

                  Hi Matt im using a commercial acrylic render but i think the fault is coming from the stainless flange warping on my chimney mount that is causing the cracks mate.
                  cheers Peter

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                  • #10
                    Re: need help with dome render cracking

                    Look into lime only mix 3-1 mix we had good results with it and we put it on while the stove was warm.

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                    • #11
                      Re: need help with dome render cracking

                      An expanding steel flue can crack the render around it. I use vermicrete around the flue to act as an expansion joint.
                      Kindled with zeal and fired with passion.

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