need help with dome render cracking

[toyota08]

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

[Balthazar]

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?

[toyota08]

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

[david s]

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.

[Balthazar]

Originally Posted by david s

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

[toyota08]

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

[david s]

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.

[aureole]

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?

[toyota08]

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