Our Dulux Protective Coatings customers often have questions in common, and some unique. Learn more below, or Contact Us for more assistance.

Hot Dip Galvanising or Zinc Rich Primer?

Hot Dip Galvanising or Zinc Rich Primer?

My client insists that the steelwork on his beachside project be hot dip galvanised before painting, but I’m concerned about painting over galvanising and would prefer to paint over a compatible zinc-rich primer. He is more concerned about long-term corrosion. Do you have any information that I can give my client to show that a zinc-rich primer can offer at least equivalent corrosion protection to hot dip galvanising?

A good first start is the Australian Standard 2312, “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings”. In it are many coating systems (including both unpainted and painted hot dip galvanising systems) and their estimated durability – years to first maintenance. Table 5.2 shows hot dip galvanised steel unpainted and painted, whilst Table 6.3 shows various primer systems, unpainted and painted. For example, in a very high marine environment, system HDG600 (85 microns of zinc metal) estimates 5-15 years to first maintenance, whereas system IZS2 (75 microns of inorganic zinc silicate coating, water borne) estimates 10-15 years to first maintenance. According to AS2312, both systems would offer enhanced corrosion protection if coated correctly. Protective coating manufacturers, particularly members of the Zinc Rich Coatings Council (link to www.zrcc.asn.au), will certainly support the view however, that their products should be applied over their own zinc-rich primers, rather than over hot dip galvanising, which is essentially someone else’s zinc coating. Our HDG Tech Notes (link to Tech Notes page) cover just about every issue of galvanising, including different types available, troubleshooting system problems, long-term performance issues and sustainability.

Can I apply a two-pack over a single-pack?

Can I apply a two-pack over a single-pack?

It depends on so many variables that there is no simple answer! Much depends on the original single-pack coating: is it an enamel or acrylic? High build or thin film? Are there signs of deterioration, adhesion loss or chalking? If the substrate is firm and dimensionally stable (unlike timber, which tends to grain-crack and warp) and if the single pack is hard, well-adhering and in excellent condition, then the answer is ... maybe! But do a test area first to see if the single-pack is compatible with the two-pack. After the two-pack has cured, perform a cross hatch adhesion test to ensure that there is adequate adhesion. If the test area seems OK, then you can go ahead.

Can I use Mixed Paint After Its Pot Life?

Can I use Mixed Paint After Its Pot Life?

To use up mixed paint after its pot life, can I add extra base, mix it up and refrigerate, then add the extra hardener (catalyst) the next day?

No! All you are doing is wasting good base and hardener by mixing it with old material that has polymerised and is no longer chemically active. The likely results of applying any coating after its pot life is expired are: delamination, grittiness or seediness of the finished film or frying of subsequent coatings. Never add new material to old material. Read more.

Can I Extend Pot Life By Thinning?

Can I Extend Pot Life By Thinning?

No! Adding thinner will lower viscosity, but will not extend pot life in any way. Diluting your mixture may, however, mask the point at which your mixture has exceeded its pot life. You will stall polymerisation, which will lead to other defects. If the mixture has too much thinner in it, then the reaction will not progress through to completeness to give the desired film properties. Your thinned coating is also likely to be too low in film build, causing further problems. Read more.

Can I Use The Part B From A Different Product?

Can I Use The Part B From A Different Product?

No! All products are formulated specifically to produce a finished product. The “Part A” in each two-pack kit specifically requires the exact amount of hardener present in its own “Part B” in order to properly cure; different products will have differing amounts of curing agent relative to base, even where the mix ratios seem to be the same. If any part of the formulation is changed or tampered with, correct curing will not be achieved, and the product will be either too cheesy or too hard. Manufacturers ensure that for a given two-pack kit, when the entire contents of Part A are mixed with the entire contents of Part B, you have the correct ratio of resin to curing agent. Read more.

Can I Use a Different Supplier’s Part B?

Can I Use a Different Supplier’s Part B?

No! Each product is carefully balanced with regard to amount of resin in the base, and amount of curing agent in the hardener. Even if two-packs from different suppliers have the same mixing ratio (for example, a 4:1 ratio) the level of hardener to resin can be completely different. The “Part A” in each two-pack kit specifically requires the exact amount of hardener present in its own “Part B” in order to properly cure. In fact, the same principle applies to different products with the same mixing ratio from the same supplier! Therefore, if you attempt to mix apparently similar products from different suppliers, you run the very high risk of application or coating failure due to incorrect resin/curing agent ratios. Read more.

If I increase the amount of hardener, will the coating dry quicker?

If I increase the amount of hardener, will the coating dry quicker?

Yes, but off-ratio mixing is always a bad idea. Apart from shortening the pot-life, other problems are likely to result, such as patchy gloss, lower gloss than specified on the data sheet, a significantly harder coating than desired (a too-hard coating can be brittle, or crazed, and easily scratched) or, conversely, a softer coating (too cheesy and easily damaged). Off-ratio mixing usually results in very poor bonding to the substrate.

Can I Paint Over Blue Steel?

Can I Paint Over Blue Steel?

The steel for our structural steelwork has been delivered with what appears to be a very hard, smooth, bluish coating. Can we apply the coating system directly over it?

No! The bluish layer is called millscale, and is a type of iron oxide that is formed on the surface of the steel during the hot-rolling process. The very high surface temperature combined with high roller pressures result in a smooth, bluish grey surface. Whilst millscale appears to be sound and firmly adhering, the underlying steel will corrode preferentially to the millscale, causing delamination, taking the protective coating with it. The only way to prepare steel prior to applying a protective coating is to remove all surface contaminants including millscale, and create a surface profile on the steel to achieve maximum bonding between the steel and the first coat. The preferred way to achieve this is abrasive blast cleaning according to AS1627.4 Class 2.5 (SA 2 _). Read more.

Can I Use A Rust Converter To Save Time?

Can I Use A Rust Converter To Save Time?

I have to paint several rusted steel sections, and I wish to use a rust converter to save time and effort. What can you tell me about rust converters? Which is the best type to use?

Suppliers of “rust converter” products claim that there is no need to remove the rust, potentially saving the applicator a great deal of work in removing rust mechanically. These suppliers also make the claims that the conversion products adhere tightly to the steel to form a protective barrier that prevents further corrosion and also provides a suitable surface to paint. But, according to Standards Australia’s “Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings”, AS/NZ2312:2002, “There is a considerable amount of published literature which refutes such claims, stating that where any conversion may occur, complete penetration of rust and reaction with it is unlikely.” Standards Australia declares that with some ‘rust converters’, only a colour change occurs, whilst in the case of phosphoric acid-based rust converters, the phosphoric acid has little or no reaction with hydrated ferric oxide under normal conditions, and that unreacted acid can become trapped beneath subsequently applied paint. The Standard states that the use of a ‘rust converter’ with any coating systems quoted in AS2312 Table 6.3 (coating systems for direct application to hand-cleaned and power-tool cleaned rusty steel) is not recommended, and will detract from their subsequent performance. Read more.

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Tech Notes

Our useful Tech Notes cover many interesting coating issues. To browse, click here.