Longmont, CO new WFO build - casted over sand

[david s]

Flexible Pointing is an acrylic product designed to go over the mortar on the ridge cap on tiled roofs. It is extremely flexible to cope with the constant expansion and contraction of the roof and as such makes an ideal solution for a pizza oven. It has high build up because it contains a high proportion of sand. It is designed to be troweled on but I find it better to water it down 20% and apply by brush, around 3 coats gives a thickness of about 2 mm.

[cnegrelli]

I'm not sure whether your question refers to the acrylic coating over the cement render/stucco or whether you mean the sand in the actual cement render itself. I actually use flexible pointing which is a product designed for ridge caps on tiled roofs. It is a very flexible acrylic with sand for high build up. If you meant the sand in the cement render then it should have a variety of grain sizes.

I already did a base coat of cement stucco over lath covered hardiebacker board. I am looking to do an acrylic coating over that and assumed you were talking about an acrylic stucco. What is the flexible pointing product you use?

[david s]

I'm not sure whether your question refers to the acrylic coating over the cement render/stucco or whether you mean the sand in the actual cement render itself. I actually use flexible pointing which is a product designed for ridge caps on tiled roofs. It is a very flexible acrylic with sand for high build up. If you meant the sand in the cement render then it should have a variety of grain sizes.

[cnegrelli]

I water down the acrylic 20% so it can be painted on. Using a paintbrush. Around three coats gives a thickness of about 2 mm. I don't scratch the rendered coat it is applied on to.

David,
Is there a certain type you use in terms of how coarse of stucco you use for this method? I notice they have sand coats (finest) and then more coarse. The one I have seems like a medium one.

[cnegrelli]

Do you use a small banjo peel for turning and unloading pizza? I use a large wood peel for loading but after that it is a small 8-9" diameter turning peel to turn, dome and unload.

Definitely need to find a good 8" peel for that.

[david s]

Do you use any sort of conditioner or primer between base coat and the acrylics? I'm going to do a base coat soon of cement based stucco (I've attached the lath to hardiebacker). I will wrap that in cling-wrap for the recommended cure time and once all water is out (I think I read 2 weeks) and it's good and dry, I plan to try your paint on stucco method of the colored acrylic I have on hand. I figure everything I've followed from you has worked out so far, so why not?

By the way, we have really zeroed in on the right recipes and techniques, and the oven performs perfectly. I am still hoping to get a door fabricated here soon and also have a lot of finish work to do. Could use some more tools (for example I can in no way cook 2 pies at a time in my 32" ID oven). But maybe with a smaller peel and more practice?

No I don't use any sort of conditioner. Just make sure that it's quite dry before painting on the acrylic or it can blister if applied over a damp surface.

[UtahBeehiver]

Do you use a small banjo peel for turning and unloading pizza? I use a large wood peel for loading but after that it is a small 8-9" diameter turning peel to turn, dome and unload.

[cnegrelli]

I water down the acrylic 20% so it can be painted on. Using a paintbrush. Around three coats gives a thickness of about 2 mm. I don't scratch the rendered coat it is applied on to.

Do you use any sort of conditioner or primer between base coat and the acrylics? I'm going to do a base coat soon of cement based stucco (I've attached the lath to hardiebacker). I will wrap that in cling-wrap for the recommended cure time and once all water is out (I think I read 2 weeks) and it's good and dry, I plan to try your paint on stucco method of the colored acrylic I have on hand. I figure everything I've followed from you has worked out so far, so why not?

By the way, we have really zeroed in on the right recipes and techniques, and the oven performs perfectly. I am still hoping to get a door fabricated here soon and also have a lot of finish work to do. Could use some more tools (for example I can in no way cook 2 pies at a time in my 32" ID oven). But maybe with a smaller peel and more practice?

[david s]

I water down the acrylic 20% so it can be painted on. Using a paintbrush. Around three coats gives a thickness of about 2 mm. I don't scratch the rendered coat it is applied on to.

[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.

David - how do you apply the acrylic coats, in terms of a trowel? I've read that a plastic trowel is preferred?

Also - on your single cement render coat, do you scratch it to better hold the acrylic layers to come?

[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....rt/tds-sheets/
choosing: XB9500-TDS-160720

[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]

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?

[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.

[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.