Longmont, CO new WFO build - casted over sand

[cnegrelli]

I like to add another coat over the cement stucco which covers any troweling marks and also fills cracks. I use a flexible acrylic product. A 100% acrylic render works well. I find it's easiest to apply by watering it down 20% then you can brush it on. Because it contains sand you get a high build coat.

David - what type of lath do you use and also is that acrylic coat a 3rd or 4th coat? I've been reading:
1. lath
2. scratch coat
3. brown coat
4. finish coat

and I presume the Acrylic is then on top of that or is it just the finish coat (traditional)?

[david s]

I break with convention here and don't use lath. Primarily because it takes too long. Instead I use randomly mixed AR glass fibres in the render coat. Also because of the time factor I do the cement render in one single coat of around 12-15 mm thick, finished with a sponge. Then wrap the whole thing in cling wrap to keep the moisture in the outer layer, letting it cure for a week to enhance strength. After unwrapping it and some more fires to ensure the outer shell is thoroughly dry I then apply the acrylic coats.

[cnegrelli]

I am intrigued as usual. And so my next question is proportions of fibres and size. Seems like for concrete the 3/4" size is recommended (for counters for example) around here.

[david s]

Yes that's the stuff, make sure you get AR, alkaline resistant fibres. I add 4% by weight. Check this link

http://www.concretecountertopinstitute.com/blog/2013/04/glass-fibers-an-essential-component-of-gfrc-concrete-countertops/

[cnegrelli]

Do you use anything at the bottom, where the stucco meets the hearth?

[david s]

Yes I cut some shallow grooves with the angle grinder and diamond blade, into the supporting slab and dampen them before doing the stucco.

[cnegrelli]

Appears there is no more residual water stored in my CaSi board (at least was my thesis). I had the oven hot yesterday for hours and no sign of the water seepage down thru the foamglas to the concrete hearth. I will be putting that panel of my enclosure back up today and adding the loose perlite (I hope).

[cnegrelli]

Cleaned out the oven today and took a brush to the inside of dome again.
Those casting ovens in the future:
Don't forget the wet newspaper. This will keep you from having sand stuck to the refractory. Sandy pizza is not a favorite of anyone! David S. warned me.....

Today I taped my joints of the vertical walls and sealed with Versabond Thinset.
I also added 6 cu ft of Perlite around dome. This gave me at least 12" high all the way around. I wasn't happy with my ability to secure the insulation blanket around the firebrick, since I hadn't cut the firebrick in the shape of the dome. So getting 4" of perlite around the firebrick (in some places more) and well up the oven dome which has 6" of blanket makes me feel that the insulation is more than adequate now.

Also got some of the perlite up on the flue area just below the chimney anchor plate, where I didn't have good coverage of the FB blanket.

Can't find the hydraulic steam vent I bought on Ebay when I was planning a dome. Decided it was overkill anyway and bought a small 2" vent and cut opening in sidewall of hardieboard.

Was thinking of using redguard on my hardiebacker since I have a flat section of roof and it's really not a good design for rain/snow. But like a shower, it would run down the outside of the cement board, but no where to go at the bottom but to pool. Would have to have a sloped counter and some sort of drainage to have it be effective. Anyone ever thought this thru for a dome design? I have the redguard already from my bathroom remodel.

[david s]

I think it is wise to slope the supporting slab slightly to the outsides to prevent/reduce the tendency for water to pool there and have often recommended it.

[cnegrelli]

I break with convention here and don't use lath. Primarily because it takes too long. Instead I use randomly mixed AR glass fibres in the render coat. Also because of the time factor I do the cement render in one single coat of around 12-15 mm thick, finished with a sponge. Then wrap the whole thing in cling wrap to keep the moisture in the outer layer, letting it cure for a week to enhance strength. After unwrapping it and some more fires to ensure the outer shell is thoroughly dry I then apply the acrylic coats.

If I understand this correctly, you do one cement render coat that has the AR glass fibers in it. Then you do multiple finish coats (after the render coat is dry) using acrylic stucco?

Do you (or have you ever considered) use a fiberglass mesh trowled into the base coat?

[david s]

Yes, but I find the best way to apply the acrylic is to water it down 20% and paint it on in successive coats sanding in between. This way you can get a perfectly spherical surface. Be careful, some products are labelled acrylic render when in fact they are acrylic modified cement renders. What you need is a 100% acrylic render that comes wet in a bucket. If it's dry in bags it will be basically a cement render.

No I've not used or seen a fibreglass mesh that is used in conjunction with render. It would have to be AR or the alkalinity in the cement will attack and corrode the fibres. The beauty of the random fibres is that you just throw them in during mixing saving a lot of time that is required when placing mesh.

[cnegrelli]

Thanks. As I am not doing a spherical shape, but have corners and edges (i.e. cement board) I've got a slightly different objective.
Would you try your approach if you had this shape rather than spherical? Please see post #121. My framing makes it tough to get corner bead at the edges using traditional lath methods.

[cnegrelli]

No I've not used or seen a fibreglass mesh that is used in conjunction with render. It would have to be AR or the alkalinity in the cement will attack and corrode the fibres. The beauty of the random fibres is that you just throw them in during mixing saving a lot of time that is required when placing mesh.

This is a link re: the mesh. I think it's the same idea as adding fibers into the base coat itself, but probably more difficult! I wonder though if it would help out more on the corners?

[david s]

If rendering on to cement board or similar, I wouldn't be bothering about adding the random fibres, but follow conventional methods for your corners.

[cnegrelli]

I have been finally getting back to the finish stage. Oven is working great. I have been adding lath to the cement board outer shell in preparation for stucco. This is a lot bigger job than I had imagined and why I spent so much time looking for a way to avoid it!

I am having trouble locating a suitable caulk to go between the stucco and double-wall chimney. What do people use here? The temperature of the outside of the pipe gets a bit hot, certainly over 120F which is what the max rating is for the silicone caulks I looked at.

I need it to be waterproof and hold up to sunlight.

edit: Found the caulk that the replacement windows contractors used. XtraBond9500, and the datasheet states it remains flexible up to 425 degrees F.
Datsheet (I couldn't seem to get the pdf to link) can be found at:

http://www.premierbuildingsolutions.net/support/tds-sheets/

choosing: XB9500-TDS-160720