Water-based masonry primers

Water-based primers have been gaining popularity for some time now- not only for environmental concerns, but also as a cost consideration. The cost consideration is unfortunate, but a real issue in the market we service. One disadvantage that a water-based primer has compared to a solvent-based masonry primer is the limited penetration into the substrate. Water-based binders are emulsified, and the droplets have a measurable particle size, meaning that penetration is not possible into interstitial spaces smaller than the droplet size. As long as the substrate is well bound,dry and not friable, water-based primers perform well. An experienced contractor will be able to make the call as to whether the substrate is suitable to be primed with a water-based primer or not, whereas decisions that are solely price driven can result in unnecessary failures. Recently I saw an instance where a still wet plaster-skim had been primed with a water-based primer and over-coated with a washable sheen acrylic, with predictably disastrous consequences. The entire coating system could be fairly effortlessly (fortunately) peeled off the substrate. The back of the paint film was entirely covered with loosely bound skim-plaster, and the moisture level was measured between 19 and 22%. The moral of the story is to apply all masonry primers only to dry substrates, but water-based primers especially only to dry and non-friable substrates.

Wet and dry film thickness

From the previous two posts, we saw how to calculate wet film thickness from the recommended spread rate. The relationship between wet film thickness and dry film thickness is dependant on a formulation parameter known as Volume Solids. This value, given as a percentage, is usually found on the Technical Data Sheet of the product. A Volume Solids of 40% means that when the paint dries
40% remains on the surface forming the dry paint film. So if the wet paint film thickness was 125 microns and the Volume Solids of the paint applied was 40%, then the dry film thickness will be 50 microns (125 x 40/100).
Two coats, applied at 8 square metres per litre, of a paint formulated at 40% Volume Solids, gives a dry film thickness of 100 microns.
Applying a product at the manufacturer's recommended spread rate ensures that the correct dry film thickness results and the performance of the coating is maximised. Thinning the coating by addition of water or solvent, or spreading the coating too far, only results in a thinner dry film - and the life expectancy of the system is reduced.

More on Spread Rates and Film Thickness

The relationship between spread rate - given in square metres per litre - and wet film thickness is fixed. Applying paint at a spread rate of 10 square metres per litre gives a wet film thickness of 100 microns. This obviously assumes that the paint is evenly applied to a smooth surface. To calculate the wet film thickness from the spread rate simply divide 1000 by the spread rate. So a spread rate of 8 square metres per litre gives a wet film thickness of 125 microns.
When paint dries, water or solvent (depending on whether the paint applied is water-based or solvent-based) evaporates leaving a dry paint film. The relationship between wet film thickness and dry film thickness is not fixed and will be dealt with in the next post.

Spread rate and film thickness

Manufacturer's will recommend that products are applied at a specific spread rate. This is literally the area covered by one coat per litre of paint. This information is given on the Technical Data Sheet of that particular product. Applying the paint at the recommended spread rate ensures that the paint dries to an acceptable film thickness. Applying the correct amount of coats of paint then gives the correct film thickness of the system. This dry paint film protects the surface coated and will live up to the life expectancy as long as no deviation from these recommendations have been made. Two coats of a good exterior coating, applied at the correct spread rate, will provide a paint film thickness about 100 microns. This is only one tenth of a millimetre - and much is expected of it. Given that acrylics weather naturally at a rate of around 8 microns per year when exposed to u.v. light and the elements, it doesn't make much sense to skimp. Using a skilled painting contractor, familiar with the product to be used, will ensure that the product you have paid for lasts as long as you had expected it to and saves you a lot of money in the long term.

The PVA confusion

The term "PVA" is often still used to describe a water-based paint. This goes back to the post second world-war era when it was the first binder type to be used in the production of water-based coatings. It had many shortcomings as a binder and was soon super-ceded by Pure Acrylics and Modified Acrylic binders, now still used as the main binders used in water-based coatings. Ask your paint manufacturer or stockist for the binder type used in the product you are about to purchase, especially if you see the term "PVA" used in the product name. A simple rule of thumb is that Modified Acrylic binders are used to produce matt or textured finish paints while Pure Acrylic binders are used to produce sheen and roof paints.

Galvanized iron roof sheets

New galvanized iron roof sheets are best left to weather for a full year before coating. This, however, is not always possible in practice. Should the roof sheets need to be coated within this time period, or if a water-proofing system is to be applied over a portion of the area, it is essential to first thoroughly degrease the area to be coated. This removes the residual oily layer present on the new roof sheets when supplied. There are many products available that will successfully accomplish this - we have an eco-friendly water-based product called Tornado Degreaser on our range that is ideal for this application. This will ensure the good adhesion to the substrate of the coating to be applied.

Wine Cellars - Specification file for all buidings

One of the services we offer to wine cellars is the compilation of a file of specifications for the re-coating of all buildings on the estate. This gives the Maintenance Manager all of the surface preparation and coating instructions,  product requirements and quantities for all of the buildings from the main homestead, managers house, labourer's cottages through to the cellars. This can be used any time in the future for maintenance budgeting and planning. This is done free of charge and with no obligation.

Wine and fruit industry coatings

Now that the harvest time is over, maintenance starts on wine cellars and in fruit packaging warehouses. We have an impressive portfolio of customers in these industries and offer a full package of advice, training and products approved for use. Large amounts of money can be saved by good planning and routine coating maintenance. We service the entire Western Cape and have references from top clients. As an additional service we keep full records of all products used for HACCP, GLOBAL GAP and ISO inspections. We are listed as a supplier in the Wineland Directory 2014 edition due to go into circulation 1 July.

Joint Sealer

This product was developed during 2013 at the request of a mass housing developer constructing panelled houses in the Western Cape. It is used to seal the joint between panels, and the main requirements are good adhesion to the substrate, flexibility and high film build. Essentially, it is a high solids acrylic formulated on a blend of binders to provide the required properties. As with all products of this type, there is a degree of shrinkage on drying, necessitating two "coats" to be spackled into the joint. Since the successful introduction of the product, contractors have used it to fill plaster cracks with equally good results. Its toughness and flexibility would make it a good option to seal between timber panels used to construct timber homes, in addition to the above applications.

Coating of cement floors

After inspecting a floor for an applicator to determine its suitability for coating, I feel that putting down a couple of essential pointers would be appropriate. Previously uncoated cement screed floors need to be fully cured, dry, free of oil or dirt, and of a sufficiently rough profile before coating. Often calls come in from applicators or home owners after the coating has been applied asking why problems have occurred - all of which could have been prevented had these guidelines been followed.  Cement slab driveways and verandas (stoeps) are often thrown without a damp proof course below them - here the rule is simple - do not coat them.
Allow at least 28 days for the cement topping to cure, then check the moisture level with an appropriate moisture meter. The moisture level should ideally be below 12% as measured by  a Protimeter. Ensure that the profile of the surface resembles 100 grit sand-paper, if not mechanically abrade the floor using a floor grinder. This step will often remove surface dirt and oil at the same time. All dust should then be removed by sweeping or vacuuming the surface clean.
The first coat can now be applied - preferably thinned 10% by addition of the relevant solvent. Generally, a 24 hour drying time should be allowed before application of the first unthinned top-coat. After another 24 hours, the second top-coat can be applied. Depending on the coating type, there will be a specific time that needs to be allowed before the coating is subjected to heavy traffic.
Coating types vary from Water-based Acrylics to single pack Alkyd and Polyurethane-Alkyd enamels, then on to the Twin Pack Epoxy and Polyurethanes. There are cost and performance advantages related to each of the above options - check with your supplier as to which one best suits your requirements.

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