I have several hundred square feet of stucco finish planned for the exterior of my house. Walls will be steel SIPs. Traditional stucco would require paper-backed lath, then the normal three coats. So I am trying to explore various alternatives to stucco which would either cut out some of those steps or save some cost, or both.

There are synthetic products which get sprayed on like a thick paint. This is not a great option. I would think all the seams would telegraph, and I'm not so sure it would look much like stucco.

Hardi-panel stucco is another option. But Hardi products seem to be expensive, and there are the seams to deal with.

In commercial construction I am seeing a lot of Georgia Pacific Densglass exterior gypsum sheathing being used. Then they use a two part synthetic stucco with fiberglass or nylon mesh in the first coat and then a finish coat.

I have read about people using paper backed lath and applying a traditional cementitous stucco coat first, and finishing with a synhetic top coat.

I still wonder why some sort of modified direct applied system wouldn't work. First a synthetic basecoat like STO with fiberglass or nylon mesh, and then the finish coat. This is often applied to foam, for example ICFs can be done this way. Maybe it wouldn't be good to apply it directly to the steel panels, but maybe with the right layer of wrap or felt?

Any ideas?

Jelly;

If you want something that looks like real stucco, use real stucco.
Nothing comes close to the real deal.

Stucoflex is just elastomeric paint with sand added, it looks great on a 2ft x 2ft. panel sample, but gets real ugly when applied over panel, channels, fasteners, etc. .....you can see every single screw.

The most cost effective with the best results are paper backed lath, 7/8" cement coating costing about $3.00 sq. ft. in our area

Jelly,

I agree with Chris. Do it right the first time and forget about it. For metal panels I like the hard shell the cement stucco gives the panel. This will save you damage in a wind storm, like penetrations, dents and tears. Also it adds mass to dampen exterior sound sources.

Metal panel buildings do what I call "breath" in other words they expand and contract with temperature swings. The synthetic stucco systems I have seen can not handle the movement over time at the panel joints. They eventually telegraph through the finish. Also, as Chris said, the channels and fasteners at the top, bottom and corners of the panel walls are very difficult to hide with a thin system. Trim of some kind can do it, but for the average home this would be very labor intensive and costly.

I have yet to solve the problem of synthetic stucco systems over metal panels. But one possibility might involve pre-coating each panel prior to installation and thus expressing the joints between the panels and not using a bottom or top channel. This kind of configuration would require a timber or metal frame for the panel support. Kingspan sells metal panels with a simulated stone finish for this kind of installation.

I hear you both, and I understand the merits of real stucco. But just entertain me for a little longer and help me understand :)

When I say synthetics I'm not thinking Stucoflex, which in my opinion would be a horrible solution.

Rather I mean STO or various other polymer-modified quasi cementitious stuccos. Wouldn't it make sense to use a traditional scratch coat and then STO as the finish coat? The idea is that the polymers would make it elastomeric and cover the inevitable cracks.

Jelly;

I used the STO product on my office addition, It appeared to be working at first
but delaminated at the joints.
"various other polymer-modified quasi cementitious stuccos" sounds like a great high tech solution, but you really need a different substrate rather than the steel skin. By the time you go thru allthose additional steps the conventional stucco is the least expensive solution with proven results. All the rest is experimentation, which is why I try it on my own stuff first.
Grail coat does not work either

Chris, is that materials and labor, or just material cost?

$3.00 labor and material, scaffold tools, etc.

Maybe I'll do that on the front... and spray paint the back   ;)

Oh! Jelly that makes my day, LOL.

You know, the secret to a good cement stucco finish is to let it crack. That means letting each coat of stucco dry properly before the application of the next. The first coat goes on dries and forms stress cracks and then stabilizes. Then you put the next coat on and it fills those cracks in, then it drys and forms smaller stress cracks and then stabilizes. So by the time you get to the finish coat, you shouldn't get any cracks or only hairline cracks that can't really be seen and can be covered with paint. So, systems that try to bypass this method with only one coat processes are extremely installation sensitive and are not likely to be lasting because they try to be elastic and continuously obsorb the stress which eventually makes them fail. The rigid stucco is a hard stabilized seamless shell that sort of floats over the wall due to the application of the wire lath with felt backing.

This is the panel my brother was taking about:

http://www.kingspanpanels.us/Industrial/ThermalspanWall/300GS.aspx

It simulates a masonry finish but doesn't try to hide the seams, which are the trouble spots when trying to conceal on metal SIPs.

Funny, it looks like an Aluma-shield panel!  ;)

Yes it does... Kingspan has recently bought out Alumashield, which I think you already know, but they are almost exactly the same panel.

I don't see how stuccoflex or any similar product is a problem if the surface is properly prepared. Is this a special metal sips problem? I don't see the problem with mgo board that has been floated, etc.

Hughianowen;

It is a stuc o flex problem ........ Stuc o flex is a coating, it is nothing more than elastomeric paint and sand aggregate, as such,  it has no hideability. The substrate, no matter if it is metal or cement needs to be dead flat or any slight imperfection will telegraph thru, the product also has bonding issues, it will air - blister.

The 2' x 2' sample panels at trade shows look great, but put on a 10 x 50 wall it has a whole lot of uglies.

Cost.............. Stuc o flex installed is about $3.50 sq. ft. vs. real 7/8" - 3coat stucco @ $3.00 sq. ft.,  plus cost of paint  @0.50 there are  no cost savings, so why bother with the fake stuff when you can have the real deal and no problems.