[Ferro List] Brush SLip Tech Balance

[Doug Lacy]

Peter -
 The slip paste should go on over the freshly applied mix somewhere between 5 and 15 minutes after it is applied, just during after the initial set.  The second coat can be applied later in the day if it is a low-water mix, if it is not, the procedure is 48 hours to allow for full shrinkage.  This second coat should get a slip coat too.  It does help with surface pores but solves a lot of other problems with using a new style low-water mortar.

Back in the day, we started out just doing a single coat with a standard float finish.  This means that we use a green sponge float.  This is the 3 dollar disposable tool that looks like a trowel but has an inch of green sponge material glued onto surface.  The sponge is dipped in clean water and the excess water is shook out and the sand is cleaned out well every minute or so.  This is very important that the excess water be shook out.  Floating provides the shell with its first drink of water after its initial set or initial crystal reaction that eats water.  This is why you see the shell go through this rapid dry phase about 8 to 20 minutes after it is applied depending on humidity, wind and sun.  This first drink of water is important but what is more important is that the shell is not starved of water after this initial set.  With porous less dense standard mixes, keeping up on the water misting and such is more important than with the dense hp mixes because the initial hydration set combined with the porous matrix translates into a bigger chance that the shell will loose too much moisture.

Using a green float would wear out the floats too fast because we are doing uneven or undulating surface texture and it takes some experience and guidance to use a float properly.  One of the problems caused by not using experienced Union grade plasterers, besides wearing out the floats too quickly because the edges are used instead of the whole flat, is that people with less than a few years of experience would apply too much water.  This could be due to getting behind and the shell setting up and becoming hard to float or just because too much water was on the sponge.  This makes a dusty crumbly surface sometimes.  

Also, adding too much water with the float causes the super P to react through the entire shell.  This causes the shell to take on that oozing molten charactoristic to a higher degree.  A little of this effect is good because it cause the mix that pushes through the lath eyelets to then droop and sag downward to swallow the lath up to a large degree on the other side.  This seems to happen anyway if the mix is right and you don't need to add more water with the float to make this happen.  You can tell how much SPJ is used because the shadow of the steel below shows through.  Too much spj or too much water causes the mud to just peel slowly off the armature.  

To make this floating procedure less complicated and more appropriate for a stronger less pourous mix, and to overcome the problems associated with adding too much water to a low-water shell or causing the shadow of the steel to show, we have come up with a brush slip technique.


You can accomplish the same, knock down sand texture and work and smooth the surface of the plaster with a fresh slip of cement powder and water stirred until a thickish creamy texture.  Useing the slip prevents adding too much water, to a large degree especially if the cement paste is fresh and not rehydrated too many times.  Using the cement slip gives you a larger window of time because you can put it on when it would be too soon for a water brush or float and you can put it on if the perfect time to float has past and you can still create a sandy texture and work the trowel marks out.  It also seals in the moisture.  We often use an acrylic thin-set and powdered cement slip if their is high winds or thier is a special need to create a tight seal but just powdered cement and water works good most of the time. 

 Also, the slip enables you to treat a flat floor plate shell like regular concrete.  When you pour regular concrete you can float it with a bull float to make that nice sheen finish because it pulls some water and cement to the surface and the float just glides over it.  You can't do this with a low water mix unless you brush on some slip.  Then it is the same.

Using the brush and slip technique makes finishing low-water mixes easier by sealing in the existing moisture instead of adding moisture to a delicate low-water shell and it buys you more time so a less experienced person has a better chance at finishing and curing the shell right.  

Doing two coats is a way that may make it easier for one person.  You don't have to worry so much about your thickness and your finish.  You should, however, brush that first coat with the slip because it is a thinner shell that will give up its moisture quicker.  It is important that the slip is fresh for the base coat because you don't want to cause a weak bond between the first and second coat because the slip is too wet.  So using several small mixes of slip, slim, cement paste, or whatever you want to call it with a half batch mix is the best bet for a one man show.  If you can do more after starting with this, you can ease into it.  Also, the slip helps make a nice trowel finish and gives you time to work tight details.
Doug

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