Architects memo no.
91: May 2008
the devil - is in the detail
In memo 83 we discussed the perceived merits of a ‘designed systems approach’ to coating specifications as against the ‘one paint does everything approach’ (for those vandals that consigned memo 83 to the circular filing cabinet, it is still extant on our website.)
The enormous feedback we got from this memo (at least two comments, including one from my brother) indicated that there were still some uncertainties in the profession, particularly regarding the differences between primers, sealers and undercoats. This memo will try to clarify the area but being familiar with the paint industries notoriously ‘woolly’ nomenclature, epiphanies aren’t guaranteed.
We may have to use brand names here to provide illustrations (apologies in advance).
The job of a primer is to provide long-term adhesion to the specified substrate as well as providing a surface to which the rest of the system can adhere. It is obvious that if the primer loses adhesion at any time, the total system will fail. Adhesion is achieved by complementing the chemistry of the binders in the paint with the chemistry of the substrate. Timber is rich in ‘hydroxyl’ groups, which provide a nice chemical ‘hook’ that, coupled with a degree of porosity in timber, that allows penetration and mechanical ‘keying’, gives a surface to which adhesion is relatively straightforward.
Aluminium presents a surface of more or less dense, inert aluminium oxide with little porosity. Aluminium primers, such as Resene Vinyl Etch primer, contain technology that deliberately attacks and modifies the nature of the aluminium surface such that adhesion can be achieved. E.P.D.M. rubber is almost completely inert and Resene Membrane Roofing Primer works by attacking, and attaching to, a very minor component in the sheeting.
Some substrates are so difficult to adhere to that the paint technologist may try to alter the nature of the surface by adding specific chemical ‘coupling’ agents to the paint or specify special surface preparation by such things as flame treatment.
Given that the primer achieves adhesion, it may require other properties to maintain that adhesion over its useful life. A timber primer will need to have a degree of flexibility to cope with timber movement and a steel primer must also be able to control corrosion in order to maintain the integrity of the surface to which it is adhered.
A sealer is quite tricky to define. I shall attempt to, however, and offer: “a sealer is a paint coating that treats a fault or a condition in a substrate in such a manner as to render it suitable for a further coating application” for debate.
There are four main areas of use. Some substrates are considered too weak to support a coating system; such as weathered, eroded concrete; old cement based paint; some stopping compounds. A product that can penetrate into such friable substrates and, on curing, turn that substrate into a sound surface suitable for subsequent painting is referred to as a penetrating sealer.
A second use, relating to the above but quite different, is over substrates, often composite, which exhibit varying degrees of porosity across the surface. The sealer is specified in this case to equalise the porosity and to present a more uniform surface for the subsequent coatings.
A third use is where the substrate contains an element that could be deleterious to the proposed coating system; a prime example is free lime in the cement renders. A product like Resene Limelock sits on the surface and presents a film that is virtually impenetrable by lime. The sealing in of tannins and sap stains by a stain–blocking wood primer stretches this attempt at a clear delineation!
Fourthly there are substrates, the surfaces of which are contaminated to a degree that could affect the appearance of a topcoat. Nicotine stains, water stains, smoke damage stains and graffiti may all benefit from the application of a coat of stain blocking sealer.
Resene Smooth Surface Sealer is actually a primer when used for its original purpose of adhering to difficult surfaces but a sealer when used for consolidating stopping compounds in wet areas!!
And finally undercoat! Undercoats are relatively simple beasts. Their principal job is to provide build; fill small defects etc so that the topcoat can be shown off to its best. The undercoat is often also required to contribute to the total barrier properties of the system (especially in anti- corrosive systems, where they are often referred to as intermediate coats).
It can be recognised by anyone skilled in the art (did I really just refer to paint specification as an art!) that there is a great deal of interchangeability in the foregoing. An alkyd undercoat could serve as a primer, sealer and an undercoat for timber in an interior environment. This is an attempt to provide guidelines, not a straightjacket.