What is rust?
A broad definition of rust is that it is ‘a type of corrosion which is caused by a reaction between oxygen and iron which is usually activated by water, (even humidity in the air) and then forms iron oxides.’ This appears as a reddish brown substance on the surface of the metal.
Why rust should be treated
Rust or corrosion on roofs is quite common in South Africa and has a number of detrimental effects:
- Reduces the lifespan of the sheeting
- High cost of roof replacement if left untreated
- Can cause holes and leaks and therefore damage to the building structure and it’s contents
- Looks unsightly
The correct method of treatment rust for roof sheeting is as follows:
If the correct corrosion treatment protocol is not followed, then rust can reappear within a relatively short space of time. There are no shortcuts to this process. The use of all in one primer and top coat paint or ‘rubber’ is not recommended. Using the Aquagard Rust Treatment System, corrosion can be successfully treated with the following 5 steps:
- Roof repair- It is not advisable to repair metal roofs with waterproofing membranes. Instead, identify the cause of the leak and fix accordingly. Aquagard technical staff can advise to ensure proper corrective measures are used. Aquagard products that could assist include, sponge Polyclosures, Topspeed screws, Butyl Lapseal tape and blind rivets.
- Cleaning and washing- Aquagard Rust and Etch Cleaner is an environmentally friendly treatment that is applied to roofs prior to washing. This product has three functions, namely cleaning, etching of metal surfaces and conversion of rust. Severe rusted or new galvanized sheeting should have an undiluted application. Weathered roofs in good condition and with minimal rust can be treated using a dilution of up to 1:3 with water. The product to be applied with a knapsack or garden sprayer. Length of treatment on the roof is variable, ranging from 30 minutes to overnight. The product is economical and only a light spray to wet the surfaces is required. 1 litre undiluted should cover 10m2 including profile of the sheeting. The sheeting should then be thoroughly washed preferably using a high pressure spray. Agitation with a broom is also sometimes useful. Sufficient rinsing off of the product is critical to ensure that no residue remains.
- Rust Conversion- All rust areas to be primed using Aquagard Rust Convertor Primer. This product contains a special acid which converts the rust into inert iron phosphate. Recommended method of application is by brush, usually spot primed on the rust places only. The rust will change from brown to a blue black colour.
- Etch Priming- All the rust treated and bare metal areas (where galvanized coating has worn off) to be overcoated using Aquagard Etch Primer. This should be applied quite thickly, so as to have an encapsulation effect and spread rate is approximately 5m2 per litre. On roofs which are in poor condition, it is advisable to apply a full coat or Etch Primer.
- Top coating- Apply two coats of Aquagard Roofdec acrylic roof paint to roofs by means of brush or airless spray. Coverage rate should be 4m2 per litre per coat. This includes the profile of the roof sheet.
Important tips, facts and other interesting information
- Thickness of coating is crucial for long term protection of a roof surface. Depending on the quality of the coating, weathering generally removes between 7-10 microns of paint layer per year. Therefore a single layer of watered down coating as undertaken by some unscrupulous contractors will look wonderful right after the job is done, but will start looking patchy within two years or less. An average of at least 110 microns will be required in order to give a guarantee of between 5-7 years. Acrylic coatings with a solids content of 50% will have a dry film thickness (dft) of approximately half of the wet film thickness (wft). Example: at least 100-110 microns wft will be required to achieve a dft of 50 microns. It’s almost impossible to achieve a completely even coverage even though it may look like it visually. If thicknesses are measured there will areas where it will be correct and other areas where it is either too thick or too thinly applied. Therefore an average figure is used.
- Guarantees should be based on the condition of the roof. It would be unwise for a coating supplier or contractor to offer an unrealistic guarantee to a roof which is in really poor condition unless they have no intention of honouring the guarantee. A client and contractor should always be aware of the terms of the guarantee from the coating supplier. Generally there are many terms written in by paint manufacturers, which are used as an escape route to not honour the performance of the coating. The responsibility then falls on the contractor. Most guarantees also work on a sliding scale, which diminishes each year. This doesn’t realistically offer very much value. A 10 year guarantee would require a film thickness of at least 180 microns and the roof surface would have to be in reasonable condition with minimal rust in order for a coating to genuinely last this length of time.
- Radiant heat barrier coatings are extremely useful for keeping roofs cool during the hot summer months in South Africa. Despite what may be promoted by some suppliers, the coating must be white in order to be properly effective. Colours drastically reduce the efficiency so as to be only minimally effective and not practically noticeable. Benefits of this type of coating include longevity, cooling to the building interior and reducing thermal movement of the sheeting. Aquagard Coolcote is a highly effective radiant heat barrier coating which works on the principles of reflectivity and emissivity. This can reduce the heat on the actual roof sheets by up to 45% which could mean a difference of between 2-10 degrees C cooler to the interior.
- Other types of roof coatings are also available from Aquagard, including polyurethane, epoxy and bituminous aluminium products. Contact your technical representative at Aquagard so that we can ascertain your exact requirements in order to make the best recommendation.
- Weather conditions are crucial to take into account when applying roof coatings. Weather should not be too hot or too cold as it can affect the performance of the coating. Condensation will require work to start later and stop earlier in the winter months. Spray painting cannot be undertaken if too windy as there will be excessive wastage and overspray.
- ‘Rubber products’ are actually a misnomer. There are unfortunately a proliferation of small time manufacturers producing these type of coatings and advertising them on social media as a wonder product to solve all your roof leaks problems. In reality a ‘roof rubber’ is a thickened roof paint with a higher resin or latex content which gives it more stretchability and flexibility. However, a coating applied to a metal or tiled roof doesn’t need these qualities. Instead the coating will benefit more from a higher pigment and powder filler content which will improve UV protection, which is so vital in harsh South African conditions. So ‘rubber’ doesn’t contain any rubber but instead, are just slightly modified acrylic co-polymer emulsion paints which have been manufactured by paint companies for more than 50 years. Many unsuspecting home owners have had their roofs ruined by unscrupulous or ignorant contractors applying these coatings. When used to seal screws, joints and overlaps on metal sheeting, they will over a period of time trap water under the membrane causing roof leaks and corrosion. Likewise, when applied too thickly to tiled roofs which clogs up the joints, it traps water under the coating and causes the roof to leak. This is extremely difficult to rectify and the roof may even need to be replaced.
