Cheap, Durable and Simple Painting of Galvanized Steel
Tired of paint flaking off your galvanized steel guttering, or fencing, or anything else hot-dip galvanized? There is an answer.
Years ago in my job as a mechanical engineer in a large oil refinery I read a book put out by the galvanizing industry association here in Australia. I can't remember the title, but I do remember the section on how to make paint stick to it permanently. More precisely, what sort of paints will do the trick.
If you are not after a super hard or shiny finish, there is a paint that is commonly available, cheap and durable, and comes in an almost infinite range of colours. What is this magic paint you ask? Plain old water-based external house-paint! In Australia we call it "acrylic" paint, though it isn't always a true water-borne acrylic resin. In the USA I believe it is known generically as "latex", though again this is a misnomer, as it has nothing to do with natural rubber resin (ie, real latex).
For convenience lets just call it "acrylic/latex" for now.
- just let the galvanizing weather for a few months to remove any rolling oil residue (if it is a rolled steel product)
- wash it down with water, and a little detergent if it is really dirty
- you do NOT need any primer
- one or two coats of acrylic/latex is all you need
- my experience is that it will stick on for many years (10 or more) and when it eventually does weather just wash it down and slap on another coat. It is very forgiving.
What's the secret?
- Conventional synthetic alkyd resin enamels react with the zinc over time forming a powdery white "soap" that breaks the paint bond and lets the paint peel off. Of course, it never does so evenly. There will always be bits that refuse to peel off, making re-coating tricky. Don't use synthetic enamels (ie, those that are thinned with white spirit/mineral turps)
- the water borne acrylic/latex paints use a different resin. Typically it is a vinyl ester (esters are what you get by mixing an acid and an alcohol). The common ones are cheap vinyl acetate (vinyl alcohol and acetic acid), harder vinyl acrylate (acrylic acid) and vinyl malleate (somewhere in between?). Often it is a mix of 2 or more (look for the word "co-polymer").
- the acrylic/latex self-etchs into the zinc coating and just never seems to let go.
- eventually the resins break down with sunlight and exposure, making the paint surface go chalky. Just give it a wash and re-coat.
Appendix: - Tardus's simplistic guide to paints
- all paints consist of 3 main components
- a "binder" - a resin (glue) to hold the pigment in place
- alkyd resin (common enamels)
- vinyl esters (house paint)
- epoxy resin (tough but fussy to prime and apply)
- polyurethane (tough)
- phenolic resin (very hard)
- natural vegetable oils, such as tung oil and linseed oil
- even fish oil!
- pigment, to give colour and ultraviolet resistance (without pigment, we have a varnish, and no clear finish lasts very long when exposed to sunlight. Just ask any yachting type who has "bright-work" (exposed varnished wood) on his boat
- a "vehicle", ie, something to thin the binder so you can get it on by brush, spray, etc. The vehicle normally evaporates and is lost.
- white spirit ("mineral turps"); made from crude oil
- natural turpentine oil (from the turpentine tree)
- Generally, the more exotic the resin, the more scrupulous the preparation has to be.
- Generally, the natural resins stick very well and are not fussy about surface preparation, but are not as hard and tough.
- When in doubt, paint it with acrylic/latex! The only exception might be on trim (eg, door frames) that are handled a lot. Even here, the paint companies are continually improving their high-gloss acrylic/latex "enamels". I'll be trying one next time.
- Gloss acrylic/latex works very well on wooden boats. Sure its not a car showroom finish, but looks fine ("work-boat finish") and sticks on and is easy to maintain (again, wash it down and slap on another coat).
This page tardus.net/paintGalvanized.html
Last updated: 11 Aug 2021
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