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Steel seals and closes pores. Wood, magnesium and aluminium leave open pores. Wood leaves a rougher finish than magnesium and aluminium is better than magnesium. More exposed aggregates with wood. 

Does it need beating back if its not going to shrink due to mortar composition? Only if you finish it with steel to open it up, so don't use steel. 

Consider uses and applications. Closed for damp and open for dry? Closed, then open for improved curing? This surely ties in with the putty carbonation issue with the face curing fast and preventing effective carbonation at depth, resulting in the mortar having to be kept damp for prolonged periods of time. 

Oxidation and other chemical reactions a consideration? Yes but good God do I really need to research that too or should I just accept the answer. Although I think the answer may give me more insight into the effect of iron in organic components, so to quote the immortal Bunk, "sheeeet". Yes I was right, this was a dick to investigate and may require someone with an actual education... 

Traditional plastering says wood for backing coats then apply and finish topcoat with steel and don't rub down. I can accept this, its preventing the most water ingress and should be thin enough for carbonation to occur fully at depth; I wonder if the two thin coats, lol, has something to do with this. The decline of lime knowledge was not the singular event it has previously been thought to be in my opinion, but rather the final nail in the coffin. The literature and longevity of Victorian air limes certainly seems to support this. The closer to the 20th century you get, the simpler the mortar and more prone to failure. Industrialisation, bloody idiocy if you ask me. What good did it really do, no-one wants mass produced shit stuff or to live and work like a dog in a city and the people who profited couldn't tell the difference and would probably be happier with a bloody miniature fiefdom anyway. Probably the biggest mistake humanity ever made and the polar opposite to progress. 

Can this be expanded upon for different applications. eg. would it be beneficial for the backing coat in a damp area to be steel floated and then the next coat be wood finished and the final steel. Could this manage the water ingress and internal moisture without providing a constant route for water to come through. Bonding coat or mechanical action may be required to achieve this or for the second coat to be applied whilst the first is still green. Yes, I tried it. Works. 

External render finishes? The lime will dry out without being open pore or having the laitance removed(could laitence increase the drying due to higher freelime content), so would it be benficial to apply the final coat with steel to prevent water ingress or could external condensation adversely affect this in some circumstances? I think sugar or another retarder/water retainer is probably essential for this. SOOOOOO...laitence is offically confusing me. Lime has a thin layer over the top which is more durable and dries faster, which is the very same effect having an increased amount of lime binder would have. The test was almost certainly completed with a mortar made to hydraulic mortar consistency, so far too wet and able to cause laitance. SOOOOO.... would a more traditional dryer aerial lime mortar even have laitance? 

External damp areas; sheltered vs unsheltered?  Layering and finish considerations?

Salt management mortars. Meh, child's play at this point. What type? 

Sacrificial topcoats purposefuly designed to act as a final location for salt. Scraped off after a year or two. Cheap poultice, crappy looking finish that's so popular. How to achieve this? Stronger topcoat ripping off surface from the layer behind? Poor bond? Good bond? Retain laitence and encourage in second coat/final coat for a weak point that leaves exposed aggregate? I think to achieve this the render coats would almost certainly have to be green bonded or even wet bonded and built up immediately...oh my God, can you imagine trying to convince some pink spread clown to do it like that. Nope. But that would be an effective wet on wet poultice and it might well just pop off in the freeze. Poshington posh pants will not like this proposal so perhaps limit this idea to true conservation rather than as an application on vernacular properties. The instaconservers will love it...its like magic and looks old and shit. I believe that would more than fulfill their expectations.