Putties & Mastics are materials used for gap filling, waterproofing & sealing and glazing.

Linseed putty

Linseed putty is the traditional putty for window glazing.

Make from linseed oil & whiting (chalk)
Very slow setting.
Easy to work, smooth with a wet or linseed oiled knife blade.
Best not to overpaint until set.
Do not leave unpainted too long or it will deteriorate.
Using boiled oil makes putty set faster than raw oil.
When buying, check the putty is still soft, partly hardened putty is sold too often. Just squeeze the tub.

Old half hard putty may be made usable by adding a little linseed oil and mixing.
Stiff putty can also be made workable by adding a tiny amount of water and working in well. Although this is on the face of it the wrong solvent, it still works well.

When painting linseed putty it is important to overlap the edge of the putty very slightly with paint. Failing to do this results in premature putty failure and water ingress, which may cause wood rot.

Old putty is hard and tough. It may be removed with a knife, though the going is usually slow. When it proves difficult, heating the putty softens it to some degree. A radiant IR heater heats the putty more than the glass, whereas heat guns heat both.

Alkyd putty

Alkyd putty is a linseed putty replacement

Faster drying than linseed putty.
Can be made by mixing household gloss paint & whiting (chalk powder)

Silicone sealant

Silicone has many uses, sealing round baths & sinks and as a glazing putty are the most common uses.

Permanently soft & flexible. Different levels of stiffness (modulus) are available.
Long lived in many applications
Water based formula.
Prone to mould, which is difficult to clean off. Mould resistant types are available, but have not proven effective in preventing mould.
Most give off dilute acetic acid during cure. Neutral cure types are also available.
Most silicones are rated for 260C, higher temp silicones are also available.
Food grade silicones are also available.
Not suitable for continuous immersion?
White silicone may yellow over time.
If a very fast set is needed, mix damp chalk dust into the silicone, and it will set in a couple of minutes.
Fairly easy to mould into soft rubber goods of most shapes.
Can be used to make (reusable) moulds for other materials.
Do not use building silicones for fish tank construction.
Observe 'use by' dates, or silicone may be solid in the tube, or fail to cure once applied. If newly applied silicone has no vingar smell, set may fail to occur.

Removing Silicone

Cut away as much as possible with a sharp blade. Remove remainder after soaking with silicone remover or wiping with a rag soaked with petrol or meths.

Finishing

Fugee and fugenboy moulding tools give a nice tidy finish.
Wood soaked in 50-50 washing up liquid & water can also be used to tool wet silicone.
Polypropylene can be tooled to produce a suitable shape with a feathered edge.
A wet finger can be used, but is not the best option. It produces poorer results, and may lead to earlier mould growth.

Burnt sand mastic

Made from very dry silver sand & linseed oil. May contain driers for a quicker set.
Coarser than linseed putty.
Also known as Scotch Mastic.
When made without driers, setting will be very slow and the mix should be stiff.
Bonds well to brick, stone and timber.
Burnt sand mastic was traditionally used around door & window frames, but is not in general use today.
Looks much better with masonry than modern mastics.
More elastic than cement mortar. Forms a tough skin with a flexible core.
Very long lived, very slow setting.
Some birds like to eat it while wet. Overpainting prevents this.

Butyl putty

Does not set like linseed

Acrylic

Low cost sealant.
Less durable and shorter lived than silicones.

Polyurethane

Suitable for continuous immersion, used in marine applications.
eg Sikaflex

Marine Sealants

Sealants for marine use need to be tougher and generally cost more.

Sealants often recommended in uk.rec.sailing include:

Adflex

Black, white & brown
From builder's merchants

Arc-Loyal

MS polymer
Slow curing
Good for underwater use

Evostick Nail and Seal

MS polymer
Low cost, £3.99 from B&Q
May be confused with similar named products with different formulae

Polysulphide

Low cost
B&Q used for
Good for underwater use

Sikaflex

wide range of colours

Putty & Mastic

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