Model Engine Maker

Help! => Specific Engine Help => Topic started by: vcutajar on November 15, 2014, 09:33:05 AM

Title: Etch Primer
Post by: vcutajar on November 15, 2014, 09:33:05 AM
Hi guys

I would like to tap the collective knowledge of the group.

After a break from the Corliss I am back at it again.  Next in line for finishing is the flywheel.  I would like to add some bits to it and was thinking of using aluminium because I have the right size.

I have read that one needs to use etch primer before painting but I can't get hold of small quantities for what I need.  Is there an alternative to this etch primer?  Does the paint really peel off if it is not used?  Can I use zinc chromate paint (if it is still available)?

My other option is to be done with it and use mild steel instead.

Thank you in anticipation.

Title: Re: Etch Primer
Post by: Jo on November 15, 2014, 11:39:04 AM
Hi Vince,

You should find that Etch primer is available in car spray (rattle) cans these days ;)

Title: Re: Etch Primer
Post by: Arbalest on November 15, 2014, 11:52:27 AM
Yes, Halfords normally have it. If possible sand blasting also helps the paint stick.
Title: Re: Etch Primer
Post by: Stuart on November 15, 2014, 11:53:54 AM
yes it will peel off very easily

Do they have Halfords in Malta ?

Title: Re: Etch Primer
Post by: vcutajar on November 15, 2014, 12:04:35 PM
Thanks guys for the replies.

Unfortunately we don't have Halfords here and all the primer spray cans I have seen here do not specify that they are etch primers or maybe suitable for aluminum.

Looks like the parts are going to be made with BMS after all.

Title: Re: Etch Primer
Post by: Stuart on November 15, 2014, 12:19:44 PM

Didn't think you did but here is a description of the rattle can stuff

The U-POL Acid #8 Etch Primer promotes paint adhesion to difficult substrates such as galvanised steel and aluminium. It is ideal for rub throughs in primer prior to application of colour and it features a fast drying formula

Maybe a search in your neck of the woods
I do know that Amazon do a something similar

Title: Re: Etch Primer
Post by: Ian S C on November 15, 2014, 01:15:03 PM
Vince, I'd go Zinc Chromate For Aluminium.  Humbrol produce little tins of it.  Cessna 185B rebuild, zinc chromate primer,  about 80% new skins etc.
Ian S C
Title: Re: Etch Primer
Post by: tangler on November 15, 2014, 01:27:01 PM

I defer to the expertise above but I sprayed the ally castings for my Wyvern with ordinary automobile grey primer followed by a filler primer and then a top coat.  The paint has also adhered to the couple of bronze bits and the copper fuel tank.   Admittedly that was only 6 months ago but it's OK so far.  When I had to scrape off the over spray it seemed pretty adherent, much the same as on the cast iron bits.  Preparation was just a wipe over with acetone followed by a scrub with detergent and water.

Just my tuppenceworth,

Title: Re: Etch Primer
Post by: Don1966 on November 15, 2014, 02:49:41 PM
Vince I with Ian on this the zinc chromate should work real good. Just make sure you clean the surface properly. If you have a way to bead blast it that's even better.

Title: Re: Etch Primer
Post by: Ramon on November 15, 2014, 03:50:27 PM
Hi Vince I concur with the Zinc Chromate and Grey primer can adhere quite well until perhaps it gets a chip - it's then that it begins to delaminate. A long way back I made an 'all singing etc' control line handle and carved the handle part from ali. Primed with grey primer and sprayed a deep yellow it was gifted as an award at the annual champs. Though I say it myself, with all the steel parts blackadised it did look the business. Imagine my disappointment sometime later then, when the recipient told me he had dropped it on the tarmac and the paint literally fell off the handle like a broken eggshell ::)

If it's possible - given the part - a light coating of JB Weld will really bond well (Yes, I know, I have shares in the company, ;D) - lightly rubbed down that will take any primer/top coat.

Something to be aware of too with some self etch primers is that after application the time before coating over should, up to a point, be as long as possible as apparently, application of the next coat arrests the etching capabilities - ie the longer the time the better the etching process.

Hope that's of use - Ramon

Title: Re: Etch Primer
Post by: the artfull-codger on November 15, 2014, 08:51:55 PM
   My 2 penneth worth,I've done quite a lot of castings in alloy & painted them with various paints but the best primer I've used is ici two part etch primer,then enamel afterwards,the etch primer[the most important bit] has never chipped or flaked off on alloy brass or steel, I'm led to believe that epoxy primer is even better but I've not used it, I painted some alloy plaques I cast with ''hammerite'' smooth & no primer & you could scrape it off with your fingernail but I use hammerite[smooth] after self etch priming o-k.
Title: Re: Etch Primer
Post by: vcutajar on December 24, 2014, 11:04:05 AM
In my quest to find a primer for aluminium I found this spray can.  It does not say it is an etch primer but it says it is for aluminium.  I guess time will tell.

Title: Re: Etch Primer
Post by: Allen Smithee on December 24, 2014, 02:34:15 PM
Vince, I'd go Zinc Chromate For Aluminium.  Humbrol produce little tins of it.  Cessna 185B rebuild, zinc chromate primer,  about 80% new skins etc.
Ian S C

I may be wrong, but it's my understanding that the small tins produced by Humbrol contain enamel paint that is the colour of zinc chromate primer, for use on plastic model kits. I don't believe it's actually zinc chromate primer.

Title: Re: Etch Primer
Post by: Steam Haulage on December 24, 2014, 03:43:30 PM
Am I allowed to interfere in this thread?

After more years in the UK paint industry than I care to think about I have accumulated a lot of knowledge, some of it questionable, about paint formulation, manufacture and usage.

If I am permitted I might be able to give some background and illumination about etch primers, especially on reactive metals like aluminium.

Jerry :old:
Title: Re: Etch Primer
Post by: Stuart on December 24, 2014, 05:03:45 PM
All I will say Jerry is get on with the enlightenment I for one hate painting with a passion any information that will ease my hate and loathing for that unforgiving coloured liquid will be greatly received 😳

Title: Re: Etch Primer
Post by: smfr on December 24, 2014, 06:45:38 PM
If I am permitted I might be able to give some background and illumination about etch primers, especially on reactive metals like aluminium.

Yes please, Jerry! I have an aluminium engine base to paint, and I'm interested.

Title: Re: Etch Primer
Post by: Mosey on December 24, 2014, 07:31:37 PM
Is air brush sand/abrasive (air erasor) blasting sufficient for a final finish? It sure looks good.
Title: Re: Etch Primer
Post by: Steam Haulage on December 24, 2014, 08:17:22 PM
Hi all,
Ok so I will, but not this evening. I need to have my wits about me for this.

Now I know how I'll be spending Christmas Day!


Title: Re: Etch Primer
Post by: bp on December 25, 2014, 01:08:27 AM
Regarding the painting of aluminium, I have a tale which supports the use of an etch primer.  I used to have a car with aluminium bodywork (it's irrelevant, but it was a 1951 AC 2 Litre Sports Saloon).  A previous owner had painted it without using etch primer.  Some years later when I had it, the poor thing looked leperous, the paint could be peeled off in long strips.  At the time my girlfriends father was the foreman of the paint shop, he analysed the problem and pronounced "...the silly previous owner didn't use etch primer", whereupon he gave me a gallon tin of etch primer, they were the days!!.
The problem with aluminium is that it continually oxidises unless something is done to stop oxidation, like using an etch primer or anodising.
Best of luck, and Merry Christmas to everyone!!
Title: Re: Etch Primer
Post by: Steam Haulage on December 25, 2014, 12:18:38 PM
Etch primers.
These notes might perhaps be thought of as ‘going round the houses’, but I think it necessary to provide some definitions and other information as an explanation of where I am coming from. These notes are intended to provide some background on which, perhaps further discussion can take place. Although I have worked on these paints over many years, on-and-off paintmaking as a servant of engineering is always evolving.
Definitions –
Substrate: the surface to be painted,
Cleaning: removal of heavy deposits of surface dirt, oils and greases, and soiling, usually by mechanical means: chipping, wire brushing, wire wool, Scotchbrite.
Degreasing: removal of the oils and greases which may still be present as almost invisible films. This can be achieved for example by hand wiping with suitable solvents or by vapour-phase degreasing with trichloroethylene. The best vapour-phase degreasing solvents were banned under the Montreal Protocol.
Etch, etching: chemical treatment of a substrate to passivate reactive surfaces and convert surface corrosion so that this is bound firmly to the substrate. This also ‘micro-etches’ the surface thereby increasing the available surface area and mechanical adhesion.
Pot-life: in 2-component paints such as 2K polyurethanes and 2pack etch primers, the time between mixing the 2 parts until the mixture becomes unusable, usually indicated by an increase in viscosity leading to gelation of the mixture.
Paint ingredients –
Binder: Does what it says, binds all the ingredients of the paint together enabling a film to be formed and provides the means for it all to be bound to the substrate. Binders are used in all types of paints, including powder coatings.
Driers: Only in air-drying paints to improve the drying rate. White lead (see under pigment below), provided for drying in traditional paint formulations, but its removal required the addition of driers, which are in general metallic soaps, such as calcium naphthenate. The drier balance between surface drying and through drying can be critical.
Pigment: Most obviously provides colour, but also can provide other properties like corrosion resistance. Traditionally white lead was the main white pigment but was replaced by titanium dioxide during the nineteen-fifties and sixties, with lead paints being banned almost entirely except for special purposes.
Solvent: We are all familiar with white spirit, used in most household paints until the advent of water-borne paints in the nineteen-eighties. Many other solvents are in common use in industrial paints, including xylene, butanol, MEK (butanone) where levels of atmospheric contamination can be controlled.
Extenders: Literally extend the pigment content to reduce cost. The main extenders include China clay, talc, whiting etc. Gloss of the paint can be controlled by the judicious use of extenders.
Additives: Used for their specific properties, defoamers, anti-static agents, slip-agents, anti-floating agents, wetting agents etc. etc.

There are two main types of etching primer –
so-called single-pack etch primer, and
two-pack etch primer.
Both types have the property of etching the surface of the substrate to differing degrees.
The yellow colour has no significance at all and should not be used to judge whether the paint you have is an etch primer.
Etch primers serve one main purpose, adhesion to difficult substrates. Two pack etch primers provide the best performance but at the cost of a pot-life which normally is sufficient for a single shift (8 hours) but not much more.
Single pack etch primers can provide the desirable properties of being ready for use, perhaps with some thinning and having sufficient structure and build to provide a good surface for subsequent coats. Their adhesion is adequate for medium duty purposes such as office equipment and relatively short-term exterior use with the use of a suitable finishing coat, and can perform well in model engineering.
However two-pack etch primers are all about adhesion, both initial and long-term. The history of these is shrouded in history and national rivalry. Some say these were invented by the US Navy laboratories and others, including a lab in the UK dedicated to high performance coatings and very close to the WWII aircraft industry had some evidence that they were the originators. Of course with the passage of time and various take-overs on both sides of the Atlantic it unlikely to ever be known for certain.
This formulation became common with a UK Specification DTD5555, and various others where not only the performance was specified but also the composition.
As yet no improvement in performance has been achieved except by departing to a different approach entirely.
The formulation is based on polyvinyl butyral (PVB) binder dissolved in butanol with sometimes other solvents, pigmented with zinc tetroxy-chromate (ZTC). This forms the base component of the primer.
The other component, variously known as activator or catalyst is comprised of phosphoric acid supplied as a solution in alcohols, generally butanol.
Concentrations of PVB, ZTC and phosphoric acid are chosen by the supplier so that the correct ratios of acid to ZTC and PVB are achieved in the mixture.
The activator can be supplied suitable for brushing or for spraying having different mixing ratios for each purpose.
Two pack etch primers must be applied at very low dry film thicknesses,5-10 microns.
If you are brushing then additional thinning may be necessary as a it is essential that a continuous film is applied and where spraying is employed great care is the order of the day to ensure  no holidays whatsoever. The activator/catalyst should not be used as a thinner. Before applying the next coat at least 4 hours is usually necessary. Overnight should be OK but longer might not work out so well.
On application the phosphoric acid reacts with the substrate, the ZTC and the PVB to form a tough film which adheres and also forms a phosphate layer providing the corrosion resistance of the relevant metal phosphates, such as zinc phosphate or iron phosphate.
Two pack etch primer has, surprisingly, good adhesion to structural polyesters (glass fibre).
As a final note the main departure from this primer has been for high performance to develop a system of 2-pack epoxy primer and 2-pack polyurethane finish. This and its successors providing performance only dreamt of in the past.
I hope this is useful even if it is boring and tedious but if anything is contentious or in the unlikely event please chip in. It's difficult to get 30 discontinuous years of accumulated experience into a short space.
Back to the Mulled Wine.

Happy reading
Title: Re: Etch Primer
Post by: bp on December 25, 2014, 01:15:05 PM
Crikey, Steam Haulage has provided a lot to think about.

Thought 1 - Steam Haulage, does your stuff apply to Aluminium alloys?

Hmm ...mulled wine sounds like a good idea.....
Merry Christmas
Title: Re: Etch Primer
Post by: Steam Haulage on December 25, 2014, 01:24:25 PM
Hi bp,

Yes it applies wherever aluminium is the primary alloying metal. The essential with ali is the passivation, no coating is totally impervious to moisture vapour or air so preventing the aluminium corrosion, oxidation is essential.

Title: Re: Etch Primer
Post by: Ramon on December 25, 2014, 01:32:52 PM
Thanks Jerry for a most informative and detailed post. I guess, like me, you have escaped for a while  ;)

I have always enjoyed painting so don't have quite the fear that does (understandably) affect some. That said I've had my share of disasters  :o
Something I have learnt here is that the 'substrate' is defined as the actual surface on which the first coat is laid and not the initial coat of paint whether primer or etch or undercoat which was my previous thinking.

Something I was told or read quite a long time back though can't remember when or where was the need to make the etch coat as thin as possible and to leave as long as possible after it's dry before over coating as any subsequent layer can (not will) arrest the etching process. I have always adopted that premise and passed it on to others on the basis that it can't do no harm if not exactly correct.

Can you confirm then whether or not that actually is correct?

Thanks again for taking the time to pass on your knowledge

Regards - Ramon

Title: Re: Etch Primer
Post by: Steam Haulage on December 25, 2014, 06:16:44 PM
Hello Ramon,

It is most important that the EP film is thoroughly dry! Difficult to determine outside a lab. In industry it is commonplace to stove primed components, but this is also difficult to get the stoving (low temperature say 80deg C) correct. I recall many discussions with paint shop foremen on this topic.
Experience suggests that it is better to leave until thoroughly dry rather than risk redissolving by the next coat before thoroughly dry. I'm afraid the drying time can be both too short and too long. It is clearly necessary to allow sufficient time for the acid to adequately react with the substrate and with the chromate. Once all the acid has reacted there is nothing to be gained.
Title: Re: Etch Primer
Post by: Stuart on December 28, 2014, 02:05:35 PM
Thanks Jerry for the information we asked and you supplied
Thanks again now I have to get my last remaining Brian cell to animal at the info and maybe ask a sensible question

Did I mention I hate painting  :Argue: 
Title: Re: Etch Primer
Post by: Jo on December 28, 2014, 02:56:48 PM
Yes Thanks Jerry  :ThumbsUp:

Did I mention I hate painting  :Argue: 

This is one of the reasons these aero engines are looking increasingly interesting to me: None of them, or well not much of them seem to need painting  ;D

Title: Re: Etch Primer
Post by: Stuart on December 28, 2014, 03:10:39 PM
Hmm strokes gray beard in deep thought  :old:

And yes I do look like the emote but use two crutches

Got back yesterday from the south , got the usual how di I get down the chimney Xmas morning GK,s egged on by DD1 , do they make that stuff they use on gray hair for beards ?


I still hate painting