Primers, Sealers and Undercoats Introduction
In this module we will discuss what makes sealers, primers and undercoats different from other paints. We will begin by defining the role each has to play and then discuss the needs of various substrates and the appropriate primer, sealer or undercoat for each. We will give the important features of each of the products.
Primers and sealers in particular are specialised paints and this module will show why it is so important to choose the correct one.
The painting of masonry and the issues of clear finishes and stains are covered in separate modules.
What is a primer?
A primer is a paint; a coat of paint that is applied directly to the bare substrate. The word ‘primer’ means ‘first’ and in this case it is the first coat to be applied to the timber, steel or other surface to be coated. The primer is the most important coat of paint a substrate receives. The final paint system is totally dependent on the primer during its job. The primer will only be able to perform well if the surface preparation has been done well.
•Primers are the anchors for the new paint system. There are big differences in the quality of primers on the market.
• A primer is wasted if surface preparation is not thorough.
•Take time to ensure all surfaces are thoroughly clean and free from contaminants. •Remove sharp edges from timber and steel wherever possible.
•With new work most priming may need to be done before erection. Planning is needed.
The main functions of primers
Primers have varying roles on different substrates but the main functions are:
•To provide excellent adhesion to the substrate for the new paint system. Primers therefore need to have good adhesion to the substrate themselves and to provide a surface that subsequent coats of paints can easily adhere to.
•To provide protection to the substrate until it can be topcoated. This is why it is very important to have exterior wood primers in colours that block out U.V. light.
•To inhibit moisture reaching the substrate.
•To seal the surface and prevent subsequent coats of paint sinking into the substrate and losing gloss.
•To stop stains. Special primers can help to prevent stains and tannins from bleeding through into the topcoats and ruining the finished job. This particularly applies to staining timbers such as Cedar or Redwood.
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•Some primers, particularly those for wood have built-in fungicides to help prevent mould growth.mzg
振动给料机 •To prevent corrosion of nail heads higher quality wood primers contain anti-corrosive pigments.
•To provide adhesion to non-ferrous metals and to stop these metals from corroding. •To prevent steel from rusting.
Primer composition
It is very important that primers are made using specialised binders that are compatible with the substrate and that have excellent adhesion to it. For example the alkyd binder used in Resene Wood Primer (see Data Sheet D40) is excellent over timber but will chemically decompose if used over zinc metal or fresh concrete. The resin binder actually turns to soap in these situations.
In the ideal world primers would be distinctively coloured to allow specifiers and painters to clearly recognise that a primer coat has been applied. A strongly coloured primer would also ensure that low quality painters could not escape with just a single finishing coat of paint in situations where 2-3 coats are needed for good durability. Timber primers also need to have a good loading of prime pigments to achieve good hiding power in order to block out damaging U.V. light from the timber surface.
Metal primers will contain substantial quantities of ant-corrosive pigments to chemically inhibit corrosion. Often they are cynically coloured to match the colour of rust.
Primers are usually pigmented and have a middle range PVC (pigment volume concentration) around 35-45% (see CPD module ‘Volume Solids, PVC and Hiding Power’ to learn more about this). This allows them to have spare binder resin (left over from pigment holding together duties) for adhe
sion to substrate purposes. A largish pigment content is needed to provide hiding power and to help seal off the substrate surface. This is very important on timber where the primer is also playing the role of sealer.
If you can imagine the surface of a piece of timber magnified 100 times it could be represented as the top surface of a can of drinking straws. As a primer dries the pigment particles slowly sink onto the opening of each straw and block it off.
Primers have an excess of pigment and this ensures they dry with a slightly roughened surface to aid the adhesion of subsequent coats.
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What is a sealer?
A sealer is designed to be used prior to, or in place of a primer. Sealers are special primers designed to perform some of these functions:
•Provide good adhesion
•Recondition poor substrates such as crumbly old concrete tiles. Resene Sureseal (see Data Sheet D42) is ideal for this purpose.
•Seal off any surface porosity to prevent subsequent coats from sinking in, resulting in a non-uniform finish. The porous substrate needs to be ‘filled’ or ‘sealed’. This ‘sealing’ of the porous surface evens out the porosity of the surface. Such a ‘sealed’ surface will allow the topcoats to appear more even, resulting in a more uniform gloss level over the finished surface.
•Block off stains.
•Seal waxes in the substrate.
Unfortunately the naming of sealers and primers is very loose and often products labelled as primers would be more appropriately called sealers and vice versa. For example, Resene Waterborne Smooth Surface Sealer (see Data Sheet D47a)could be described as an adhesion primer rather than
a sealer.
Sealers are required to do quite special jobs that can be quite varied. Traditionally the key role of a sealer has been to bind up porous surfaces such as crumbly concrete or gypsum plaster. In these areas a PVC of about 20-30% allows for plenty of spare binder to bind up the surface of dodgy substrates and to provide a good anchor to those surfaces. Resene Sureseal is especially strong in this area and is formulated on a urethane alkyd resin with good alkali resistance so it can be used over concrete as well as interior plasters.
Other sealers are often referred to as sanding sealers and may be formulated more like undercoats with much higher PVC’s. Talc is used in these products because it provides a lovely slipperiness that greatly aids sanding.
While the colour of a sealer may not be important, having some distinctive colour there does help to see where the sealer is being applied, so that complete coverage is achieved. Resene sealers are generally coloured to enable identification of them in the case of disputes.
Functions of sealers
Some sealer functions are very similar to that of primers. For example both need to: •Provide excellent adhesion to the substrate.中国
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信息化 •Seal off any surface porosity.
Sealers for interior paperfaced plasterboard and similar substrates are also required to: •Block off water stains (existing), e.g. Resene Sureseal.
•Help seal off stains from fire damage, e.g. Resene Sureseal and Resene Quick Dry (see Data Sheet D45).
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•Waterproof the substrate (to some extent), e.g. Resene Sureseal.
•Provide a good surface for easy sanding, e.g. Resene Broadwall Surface Prep & Seal (see Data Sheet D807). In this case the Resene Broadwall acts as an ‘anti-sealer’ and actually acts to make the entire surface more porous but evenly porous.
On exterior or interior concrete, sealers such as Resene Sureseal may be used to:
•Bind up crumbly surfaces of weathered concrete.
•Provide a barrier coat to allow the use of alkyd paints over concrete.
常艳现状On composite wood products (e.g. particle board) sealers seal off the waxes from alkyd type paints. E.g. Resene Quick Dry and Resene Particle Board Sealer (see Data Sheet D43).
Products such as Resene Qristal Poly-Flat or Resene Poly-Satin (see Data Sheet D52) are used on timber as sealers for clear finishes.
What is an undercoat?
An undercoat is applied over a sealer or primer and is designed to do the following: •Provide a good adhesion to primer or sealer.
•Provide a base that the topcoat will stick to.
•Provide film build for barrier properties to prevent moisture etc. getting to substrate, especially important for steel.
•Even out small imperfections in rough surfaces.
In these days of 100% acrylic paints, undercoats are almost obsolete on timber and galvanised steel. Instead of a special undercoat the first of the two normal finishing coats of acrylic gloss can be considered as the undercoat.
Undercoats generally have the role of filling up surface imperfections and acting as barrier coats against moisture. They are formulated at PVCs of about 45-55%. Undercoats for steel often utilise extender pigments such as micas and talcs, which have platelike shapes and are very efficient at preventing moisture passage. The high PVC of undercoats (and primers) means they will not weather well and should be topcoated as soon as possible.
Functions of undercoats
The functions of undercoats also vary according to their end use. Undercoats for timber are generally used to provide filling properties and provide a smooth even surface for the finishing coat, e.g. Resene Enamel Undercoat (see Data Sheet D44).
When acrylic paints are used the trend is to use the same acrylic as its own undercoat. This ensures good hiding power because the two coats are identical in colour. This also has the advantage of needing fewer different cans of paint.
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The waterborne enamels Resene Enamacryl (see Data Sheet D309) and Resene Lustacryl (see Data Sheet D310) and the solventborne Resene Lusta-Glo (see Data Sheet D33) should be used as their own undercoat.
Undercoats for steel play a completely different role in that they provide the major means of blocking off water and salts from the steel. They achieve this by being there in bulk and are usually formulated on very impermeable epoxy binders, e.g. Resene Armourcote 510 (see Data Sheet RA40). Film builds are normally 4-6 times thicker than timber undercoats.
The Importance of surface preparation
Just like paints and other coatings, primers and sealers perform better when the substrate is properly prepared. Cracks, nail holes and other surface imperfections should be filled and sanded smooth. Rusty surfaces, in particular, need extensive work to prepare them adequately. Most important, the surface should be clean and free of all dust, dirt, grime, loose or flaking paint and other contaminants before primer is applied.
While it is always wise to use a primer in order to achieve the highest quality paint job, the following circumstances are most important for use of a primer:-
•When painting new wood, new plaster or paperfaced plasterboard, or another surface that has never been painted.
•When repainting a surface that is uneven or badly deteriorated.
•When painting a surface that has been stripped or is worn down to the original substrate.
Primers, sealers and undercoats for exterior timber
Timber is an especially difficult substrate for four main reasons.
•It has a natural tendency to rot and grow mould.
www.csc108•When it gets wet it swells anywhere from 1-5%. This doesn’t sound much but a 2% change over a board 20 centimetres wide is a movement in paint thickness terms of 4000 μms (4mm). Remember that an average coat of paint is only 35 μms thick. The end grain of timber is the major problem because water is absorbed through end grain about 50 times faster than through the face of the timber. It is extremely important to prime timber end grains.
•Timber is made up of fibres of cellulosic type material bound together by material called lignins. These fibres (like human skin) can be zapped or badly burnt by U.V.
radiation from the sun. The surface of badly weathered timber can be likened to a bale of straw but with the binder twine cut. The surface fibres are just sitting there and if painted over, the new paint system would be expected to fail by blistering off in about 18 months time.
•Nail fasteners in timber will tend to rust in damp conditions.
•At Resene we have considered these problem areas and have a range of products carefully engineered and proven over time. We will cover these products one by one. _________________________________________________________________________________
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