Yellowing of Alkyd resin bound Enamel Paints

The binders used in High Gloss and Eggshell Enamels are called Alkyd resins. Chemically they are (vegetable) oil-modified polyesters. The percentage of oil, and the type of oil used, are responsible for a range of properties that the resulting polymer will exhibit. One of these properties is the amount of yellowing that coatings produced on these binders will show. This is particularly noticeable in white paints. The yellowing occurs faster in dark environments or in areas where cleaning materials containing "active ammonia " are used. Unfortunately there are no additives paint manufacturers can include in their formulations to reduce the level of this occurrence - the raw materials used in the binder production determine this. Alkyd resin manufacturers are, in turn, limited by vegetable oil-supply constraints as well as costs. Often water-based alternatives give better results than traditional solvent-based enamels without yellowing or becoming brittle over time.

Flexible Water-proofing Acrylics

Paint outlets often have displays showing highly flexible paint films used to waterproof flashings and parapets. While these paint films appear to be very impressive to customers, there are a few facts worth taking into account before making a purchase. High flexibility does not equal quality or ability to waterproof a structure. High levels of flexibility in a waterbased coating are achieved either by use of a 'soft' binder, or by partial replacement of the total binder loading with a rubber latex. Rubber latex containing acrylics must always be over-coated due to poor u.v. resistance properties. As u.v. light breaks down the rubber latex, the coating becomes porous and ceases to function as a waterproofing barrier. The degree of flexibility required by a water-proofing coating needs only to be sufficient to absorb the difference in expansion and contraction rates between the different substrates it is applied to. In the case of flashing, this would be the metal flasing and the plastered parapet or roofing material. Soft pure acrylic binders can absorb this type of movement, and do not require over-coating, as they are u.v. resistant. Check with your supplier as to the binder type used to produce the coating - the correct choice will save time and money in the long run.

Pre-coated roof sheets

Many new developments feature homes with pre-coated roof sheets. In some instances the colour of these roof sheets fades within a short space of time, and home-owners need to re-paint. Acrylic roof paints, provided that they are formulated on "adhesion promoted" pure acrylic binders, can be applied directly to these roof sheets without a primer. The use of an anti-corrosive primer, when it is not in direct contact with metal, is a pointless exercise - and simply adds to the cost of the job.
If the coating on the roof sheets is chalking, wiping down with Mineral Turps prior to re-coating is essential. Sanding down of any rusted areas to a bright metal finish, and spot priming only these areas with an Etch Primer, completes the surface preparation procedure.
Before using an acrylic roof paint on pre-coated roof sheets, check with your supplier whether the binder is "adhesion promoted."

Coating of rat cages

This is one of the more unusual applications we have been asked to advise on. At the end of it all, it is all about the type of substrate to be coated, and the requirements of the dry paint film, just as in any other case. Rat cages are supplied with a thin powder coated layer, with limited resistance to rat urine. After a short period of time, the powder coating is broken down and the exposed metal begins to rust.
To re-coat it is necessary to sand the substrate down to bright metal where rusted, and to an even matt finish where the powder coated layer is still intact, before applying two coats of Chlorinated Rubber Enamel at 4 hr intervals.
Chlorinated Rubber Enamels are self priming on metal surfaces due to the very good adhesion to the substrate and very low moisture and oxygen permeability. They are resistant to both acids and alkalis, and non-toxic in the dry state. To ensure that the coating has dried sufficiently before the cage is re-used, it should be left for 24 hrs after application of the second coat.

Aluminium Roof Paints

Many roofs on rural buildings are still coated with Aluminium Roof Paints. This is a tradition that goes back many decades and is still very much alive. Problems occur when roofs are coated in both very cold and very hot weather. During winter months, day-time temperatures seldom rise above 15 degrees celcius with below zero overnight temperatures being the norm. This is compounded by dew-fall on most nights. It is best to only paint between the hours of 10 am and 3 pm at this time of year. Should the coating become wet - via rain or dew - before all the solvent has evaporated and the leafing of the aluminium flakes has taken place, an uneven or patchy appearance will result.
In summer months, day-time temperatures are often above 40 degrees celcius. Application of Aluminium Roof Paints in temperatures above 30 degrees celcius result in rapid sovent flash off giving the same uneven, patchy appearance as mentioned above. Early mornings or late afternoons, provided dew is not expected, are the best times to apply these coatings in summer months.

Coating of fish ponds

Ever since koi fish ponds became popular, enquiries with regard to coating and waterproofing fish ponds have been on the increase. Paint manufacturers are often reluctant to give advice or recommend products to use in these applications - and for good reason. Manufacturers have little control over preparation and application on site and the prospect of a client's prize fish turning belly-up makes the potential risks outweigh the reward in profit several times over. There are products that are very successfully used in this application - provided that the contractor adheres rigourously to the specification provided. Chlorinated Rubber Enamels, Moisture Cured Polyurethanes and Twin Pack Polyurethanes are most often used - specifications and advice can be obtained on request.

Mass housing coatings

The mass housing market in South Africa is one of the only areas in the construction industry that remains vibrant in spite of the economic downturn. Every site has its own set of requirements and there is no universally acceptable system. We have provided coating solutions for numerous sites over the past 5 years and have experience in most major regions of South Africa. We are keen to share this experience and can benefit developers in this market.

Clear Epoxy Sealers

I had an enquiry today concerning overcoating a previously sealed cement floor with a twin pack epoxy. The client was not satisfied with an alkyd based sealer previously used and asked a contractor to apply a twin pack epoxy clear coat over the existing coating. The contractor went ahead and the alkyd based coating dissolved causing the epoxy to peel off in large areas. When using a twin pack epoxy or polyurethane over previously coated surfaces, it is essential to determine the compatibility of the two systems. In this case the solvents in the epoxy coating re-dissolved the underlying layer causing the observed failure. The alkyd-based sealer should have been completely removed prior to use of the twin pack coating. Fortunately this was a small area in a domestic home and the client felt partially responsible!!

Paint Advisors: Mark and Joanne Giddey

Introduction

The aim of this blog is to share the knowledge we have gained over the past 30 years in the paint industry in South Africa. I have been involved on the technical side – mainly formulating paint coatings – while Joanne has been in sales and technical consulting. We both are of the opinion that the man on the street has a right to know more about coatings, and that it is our responsibility, as so-called experts, to impart that knowledge.

We are based in the Western Cape and welcome any enquiries with regard to decorative paints, light industrial coatings, waterproofing and coatings for wine-cellars and the fruit-packing industry. Joanne has, in addition to this, been involved in restoration and coating of historical buildings.

Between us there is a wealth of knowledge, and we have built up a network of contacts in the industry that could be of great assistance to the public. We are eager to share this and help interested parties wherever we can.

Any paint related questions are welcome.