Longmont, CO new WFO build - casted over sand

[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]

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]

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.

[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).

[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]

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

[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.concretecountertopinstitu...e-countertops/

[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]

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 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)?

[cnegrelli]

That's my plan.
I plan to tape/thinset the hardieboard then stucco.
Do you apply a stucco mesh and then a 2-step stucco ? Then acrylic?

[david s]

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.

[cnegrelli]

Concluded attaching Hardieboard to the enclosure. Anyone know how forgiving the taped joints will be with Stucco applied? Some of my joints are perfect, others I have a bit of a joint mismatch (geometry is hard). Trying to determine if I can tape as is or if I should use some "fat mud" as tilers do to plumb up walls before application. My thought is that the stucco can cover it all up with "floating".

Inputs?

Also, because I used heavy gauge steel framing, the hardiebacker fasteners wouldn't penetrate without pre-drilling. That was a blind PITA. I ended up using some TEK self-boring screws that worked well. They do stick out a bit, will that be ok for stucco?

[cnegrelli]

Got the oven up in Temp today and also discovered the "wet issue". I have been working on my enclosure a bit and firing the oven a bit this week. Still seemed to be wet down low on the walls. Discovered that the concrete hearth was wet around the foamglas that supports the CaSi and then Firebrick. Here's what I think has been happening:

1. cast dome over sand and left wet sand inside dome for a week, while misting heavily the top of dome and keeping tarped. Water definitely dripped off dome and would hit CaSi. Because Im' doing an enclosure the firebrick floor, and the insulation under it extended beyond dome, so water collected on it. I believe the CaSil got pretty saturated on the edges.

2. after digging out the sand (and David telling me I didn't have to keep it wet like that) I let it dry out a few days, but then re-soaked the inside so that I could patch the dome in a couple places where I had voids. This likely contributed as well to more moisture collecting on the CaSi, thru the cracks in the Firebrick.

I seem to have most of the moisture out, but I'm still not full-go on the fires. Here's a pic around the FoamGlas.

[Gulf]

I do not use mine for wood storage. But, I did patch the dry stacked joints witn normal mortar and then seal mine with Drylok.