20 Apr Sheltercoat
Spencer Hall ACR
A sheltercoat is by definition a surface based breathable ‘paint’ system used to protect delicate stone and mortars. Sheltercoats are generally made up from lime based mortars using a thinning medium to create a ‘paint’ like consistency. Just as traditional lime paints use an additive such as casein or tallow to bind them, sheltercoat can be given extra adhesive qualities from the addition of skimmed milk which provides naturally occurring lactic acid casein.
Sheltercoats can be used in a variety of ways, but they are generally seen as a sacrificial coating that is used in combination with mortar repairs to help protect friable stone and fill any micro fissures (which water may penetrate). Water penetration and subsequent freeze/thaw actions lead to the majority of stone breakdown and so encouraging water run‐off (when applied correctly, water should simply bead and run off the stone) will help prolong the life of sensitive stonework.
Sheltercoats can also aid aesthetic cohesion and can on occasion therefore be used to help colour match mortar repairs, pointing or tone down areas of damage. As they are sacrificial, sheltercoats should be considered a temporary solution or seen as part of a longer term maintenance scheme.
- Sheltercoats should be considered as a consolidant as much as a protective covering.
- Stonedusts are used for pigmentation within the mix, but also have the added benefit of acting as a pozzolan aiding the ‘set’.
- It must be tended care and consideration after application, as its success is dependent on this
- Sheltercoats should not be applied too thickly, as they can if not applied correctly obscure detail that it covers. They should therefore be used judiciously and should enhance the stonework they cover, not hide it
- Use selectively, as consideration should always be given as to how it will appear from ground level ‐ as it will look very different from the scaffold. Sometimes therefore a pallete of colours can be used to create a blended more subtle finish.
- One should appreciate that over 6‐12 months, sheltercoats will soften aesthetically and as such this should be a consideration during application
- Sheltercoats should be protected from drying out too quickly when applied – this may involve shielding them from direct sunlight for a period and keeping them damp to allow a slow set
- Sheltercoats should not be applied at temperatures of 5C and falling, as they (like with all lime work) are susceptible to frost and in turn failure
- Lime (both Putty and Hydraulic limes can be used)
- Constituent materials as used for mortar repairs (stone dusts, sands etc etc)
- Skimmed milk
- Biocide or Bio-Inhibitor
- Clean Buckets
- Enough Hessian to cover over all elements. Ideally double thickness during tending
- Enough Plastic sheeting to cover all Hessian
- Fine nebulous water sprays (with a fine ‘mist’ setting)
- 4‐6” masonry brushes (smaller brushes will be necessary for hard to reach and delicate
- Paper body suits, gloves and glasses (application can be quiet messy and lime will aggravate skin and can burn eyes!)
This is by no means ‘the’ definitive approach, but this is an overview to a basic methodology for application that I use.
1. All surfaces should be cleaned and delicately de‐frassed back to a stable background – care should obviously be taken to minimise any loss of historic fabric
2. Knock up mortar repair medium working on the basis of 1 part lime to 2.5 parts stone dust/sand etc and mix well, making sure there are no lumps of lime visible.
3. Add an appropriate amount of Skimmed Milk to the mortar in a clean bucket until a smooth single cream consistency is achieved – using hands is normally the best method for this to ensure the mix is fully combined
4. Add approximately 3% biocide by volume (this will ensure that biological growths are not encouraged to re-establish over the organic ‘paint system’).
5. Pre‐wet the stone that the Sheltercoat is to be applied to, so that it is well saturated, but NOT so that it is flooded
6. Keep agitating the mix in the bucket to maintain that all aggregates remain in solution and apply by brush in multi directions working the ‘paint’ into the stone. Care should be taken to ensure that the application overlaps any exposed broken edges of the stone beneath.
7. Using hessian, dab off any large aggregates and bubbles (which may develop). Try not to scrub so that the mix is removed, but dab at it flattening the application
8. Using the finest mist spray possible, keep the area applied damp (don’t allow the jet to wash off the sheltercoat, but keep it moist with regular spraying)
9. Apply 2‐3 coats, building up a covering and allowing it to cure between coats. Try and avoid applying any application too thickly in one go
10. Cover all elements with a shroud of wetted Hessian sheet (ideally avoiding it touching the stone)
11. Cover this hessian with a plastic sheet (which will create a damp micro‐climate behind it) and allow the sheltercoat to cure slowly and not dry out too quickly.
12. Keep the surface wetted and protected over 2 or 3 days to allow a slow but stable cure
13. Once the final application has dried you should note that when the sheltercoated areas are wetted – that the water beads and runs off – this is what we want
14. Uncover and ideally view from distance to confirm aesthetic cohesion