Surface Prep. Primers and Zinc?

Questions we get asked all the time are on surface preparation and how clean does a surface have to be. The obvious answer would be to say blast cleaned white metal to SA 2.5 standard. Although correct the answer is a bit of a cop out when it comes to vehicle restoration in real time circumstances. On a vehicle some areas of steel work may be too thin or delicate to blast clean to such a degree or it may be an assembly of parts that you do not want to strip down. So what do you do in this scenario.

The first thing to decide is how clean you can get the part, If it’s a large steel component and you can either blast or power tool clean to bright metal then this is always the way to go especially if you are going to use a zinc coating which we will come to later on.

Degrease, first of all using a water based detergent degreaser like Rustbuster SP10 caustic detergent or more eko friendly Rustbuster Safer Degreaser. You may have your preferred brand but make sure  that the detergent is water based and emulsifies the grease into a foam and completely removes it from the surface. Engine cleaners like Gunk are not suitable for degreasing surfaces that have to be painted neither is a petroleum based cleaner, turps ,white spirit, petrol diesel or paraffin these will all leave an oily residue, most engine degreaser will leave an oily residue to act as a rust inhibiter this rust inhibiting oily surface will not allow paint to adhere. The degreasing process should be followed by the use of a Salt remover Rustbuster Chlor-X, I have written a whole section on salt removal please take a look at it.

Maybe we can only get the surface to a clean rust free but pitted steel which could not be described as bright metal. In this scenario it is important that we use the correct type of paint coating specifically the primer. The coating we use needs to be surface tolerant. Coatings that are described as surface tolerant have very fine pigments and small molecules with a low surface tension resin this allows the coating to sink deep into the steels surface and get a really firm grip on the surface. This is why it is important that you grind or coarse wire brush a rough surface profile to the steel to give this type of coating something to cling to. At Rustbuster we have long pioneered the use of surface tolerant coatings and in particular the use of surface tolerant Epoxies, our Epoxy 121 for example has very finely ground nano particles and a slow curing resin added to a slow curing hardener. Once mixed together and applied to a suitably prepared surface the resin is not already bound together like a single pack product, this enables it to flow or fall into pits and troughs on the surface of the steel being unhardened or still an open matrix air has time to escape from the pits and troughs. Over a period of time the molecules find each other and settle down and cross link or join into an extremely tough coating that is bonding to every square mm it touches meaning that the adhesion and strength of the coating are second to none.

What you absolutely must not do is polish the surface prior to painting. A paint cannot adhere to a polished surface even an etch primer requires some kind of surface profile to cling to. Use steel wire brush tools or clean abrasives and abrasive wheels to prepare a surface.

In an attempt to explain Zinc and its correct application I may offend some expert metallurgists with my simplicity but please bear with me.

Some would prefer to use a zinc based product over a surface tolerant epoxy, is this not a better solution? Well the simple answer is yes but loaded with caveats, only if you understand what preparation zinc requires to be able to be a better primer. I would agree a good high solids zinc rich epoxy primer is a superb product in the fight against corrosion.

Zinc is a metal, as a metal it holds a position in a scale of nobility. In this scale Gold is top, gold is a very stable metal found free in nature it does not corrode. Steel is not found free in nature we have to force it to be steel from the earth and this unnatural process means that the steel wants to revert back to what it always was, earthy rust. Zinc is slightly less noble than steel so it is more likely to corrode than steel.

So how does corrosion of Zinc stop the steel rusting ?

Without using complicated equations lets put it this way. You have a steel substrate that you have prepared to bright clean steel. Onto that steel you lay down a primer of high zinc content these metallic zinc particles have to come into good electrical contact with the steel primarily for good adhesion and electrical contact. You may then decide to apply a topcoat over the zinc coating. When a stone hits the coating and chips it right back to the bare metal the steel is exposed to the elements. Water from the atmosphere, rain etc fills that pit and the steel and the water forms an electrolyte we now have a little battery, the steel being the stronger of the two metals becomes the cathode and the zinc being weaker becomes the anode. Together they create electricity. As in any battery the anode is the part that is consumed in the process but also in this process the consumed zinc produces not rust but zinc phosphate salts. These zinc phosphate salts fill the chip hole in the coating and the corrosion process stops. This is what we call an active coating the process is cathodic protection. All of Rustbusters Zinc coatings react this way. Rustbuster Epoxy Anode, Active Zinc, Cold Galv Primer, MCU Mio-Zinc please enquire which one best suits your circumstances.

Zinc is a really cool way of protecting steel from corrosion.


You really have to have the very best mechanical and electrical contact for the zinc to be able to work to the best of its ability, this means bright clean steel with a coarse surface profile. Once applied you should protect the zinc with a high build Epoxy build coat as a protection barrier there is nothing better for this than Rustbuster Epoxy 121. On a poorly prepared surface the high zinc solids would not adhere well and the coatings adhesion and stength is likely to fail before it has time to work. That first hit with the stone could take it off.

So which is best ?

If the surface cannot be prepared to bright clean steel then the high build Rustbuster Epoxy 121 has far superior adhesion and barrier strength to a zinc coating. If you can prepare to bright clean steel then The Zinc rich epoxy primer followed by the high build epoxy is absolutely formidable.

What if your zinc primer says it can be applied to a wire brush cleaned surface or over rust or over other coatings ? Well it will protect to a degree but its adhesion,strength and cathodic abilities will be greatly reduced. You would be better off choosing the Epoxy 121 if you cannot get the surface clean for zinc. Epoxy 121 even performs well when painted over rust, not recommended but a fact.

Whatever coating type you choose, you will improve its life expectancy by years by cleaning and preparing following all of the precautions in the article. Next step check my top tip on Stripe coating.

All the best


Do it right do it once

Share this post

Recent Posts