Lime can be difficult to get to grips with and use properly, especially for DIYers who might be more used to cement. Lime putty takes a bit more understanding than cement as it sets in a different way. Hydraulic lime, though, is a bit easier to use as it works in a similar manner.
There are some common tips for lime putty though:
If using a pre-mixed lime putty (which hopefully you will be) you will need to ‘knock it up’ before use. Here you will need to use a flat bed mixer, or just manually pummel the mix to ensure that the putty is spread in an even manner over the aggregate (hopefully not sand) with a fairly consistent moisture content.
Getting the mix right can be a bit hit or miss as it will depend on the substrate, the weather, location etc etc. There is no better tool here than experience, so it might be good to talk to the lime supplier about how their particular mixes work and get some pointers as to their thoughts on your particular situation.
Lime likes to be put onto a moist surface rather than a dry one otherwise it will not adhere properly this is because the underlying substrate soaks up the moisture from the mix immediately, thus drying it too quickly and creating a dry zone between the render and the wall. Thus there is no bond.
Ideally render putty render should be thrown or pumped onto a wall. This can be difficult in a DIY setting, but I have just used my bare hands (although gloves are recommended). So set down a load of sheeting around the work area and throw it on. Working the render with a trowel can pull the lime away from the substrate and so try and keep any working of the render down to a minimum.
It is better to add layers of lime render (assuming that you are doing a scratch coat as a first layer) when the lime is still ‘green’, i.e. it has not dried out completely and the top surface is still just about workable. This stops the creation of independent layers and will create a more consistent render. If your lower layer has dried overnight (for example) then a tip is to wet down the scratch with some limewash and then add on the next layer.
To get a really smooth finish on the render (for internal work) you can use a lime plaster mix (this has smaller aggregate particles) over the top. Again you should apply this to the underlying render when it is still green. If you want a silky smooth finish (like gypsum) you can either put on a thin coat of pure lime putty or use a clay plaster.
Finish off external render with a limewash. Using limewash helps to protect the render as it effectively forms a wearing layer. However if you have used lime putty with a breathable aggregate then it should last many many years as the whole system works in harmony with itself.
Be careful with using lime as it is caustic and we would recommend consulting with an experienced lime putty professional to get the right mix etc first of all.