How to work with lime

The main difference between working with hydrated and hydraulic mortars AKA air lime and water lime AKA pure lime and NHL, is that one requires water to set and the other doesn't. Or at least at a level so insigificant as to be immaterial. 

Hydrated lime is not a very strong binder so it requires compaction for its strength or rather it requires compaction to not be prone to a variety of failures. It also requires the removal of the majoirty of its water for improved capillary action and strength. The common misunderstanding with how lime works is that the mortar or plaster should be porous when in reality its the lime itself which the moisture travels through and in fact too much porosity can hinder this as the route of pure lime which the moisture is transferred along is interrupted by voids. These voids can be present due to two things predominately, a lack of compaction and an excess of water. Compaction reduces the number of voids formed in the mortar from the introduction of air by mechanical action and steam. Excess water content leaves behind voids as it evaporates. When the mortar is in place, well compacted and allowed to dry almost fully it can then be saturated to introduce carbon deeper into in at regular intervals to promote carbonation. It should not be kept wet as this is just preventing carbonation. In a few cases this could be advised but that's another subject. So to summarise, use as dry a mortar as you can make whilst still retaining cohesion and plasticity, compact it as much as possible, allow to dry, then subject it to regular wet/dry cycles.  

Knocking back, this is woefully misunderstood or even willfully. This is to compact the mortar not remove some of it, if you are removing the surface then you are doing it wrong. A mistake I myself made when first using lime. As to why that happens; it can be a number of things eg. too wet, allowed to dry too much, laitance, too soon. 

AIR LIMES SHOULD SHRINK UNLESS YOU'RE VERY GOOD AT MAKING MORTAR. This should only be prevented by using the correct amount of water, right blend of aggregates, right lime:aggregate ratio and good mixing practices NOT by the use of fibres or active aggregates such as stone dust or pozzalons. Fibres are for applying a larger mass that does not require a high degree of water resistance, be it a plaster base and mid coat, grout for wall heartings or floor slabs. All of which form part of a monolithic structure with additional protective layers. It could even be considered beneficial to have an increased number of voids for insulative purposes in some applications. Pozzalons are for increased strength as a form of protection, eg. erosion on a chimney or a permanently damp scenario.

Using a churn brush to produce an open pore finish is something which confounds me. I can only assume its because when you use steel on lime it does close it up very tightly and could potentially produce spalling but I think the bulk of the reason stems from two sources.

1. A mistaken emphasis by the conservation community on the aesthetic appearance of mortar.

2. Because laitance can cause issues. 

However, air lime mortar should never be wet or lean enough to produce laitance. As an additional note on this; the use of Magnesium, Resin and Wooden jointing or finishing tools could alleviate any issues one might have from using steel which oxidises the surface and creates a much less breathable finish than the other materials. 

Hydraulic lime is a strong binder whose application is very similar to that of OPC mortars and concretes. It does not require compaction for strength unless using some the weaker NHL 2's  and it does need to be kept damp/wet for at least a week. Then subject ot wet/dry periods. When finishing NHL's it is important to consider laitance as this is more likely with a wetter mortar gauged at 1:3 as most manufacturers advise there NHL's be used. 

Weak NHL's are a somewhat confsuing material as they are both an air lime and NHL mortar so require a combination of the two methods which would have to be determined by the professional. But essentially you want to compact them, keep them wet for a week and then subject them to wet/dry periods. Water content, time periods etc. will be deterined by each specific application.