Painting aluminium alloy black

[Robin GrahamParticipant @robingraham42208]

I've no experience of painting ali at all – research suggests that it's tricky. So I'm after any suggestions for brands of etch primer / black gloss which people have found to work. I've looked at reviews for various products on t'internet, but there are always people who say they're rubbish and give them one star – but you have to wonder sometimes if they know what they're talking about and have used them properly. I'd be more trustful of any advice from here. I don't need a filling primer- it would be on machined bar with a reasonable finish.

Robin

[Robin GrahamParticipant @robingraham42208]

[Nick_GParticipant @nick_g]

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This is a good product for your needs. Not cheap but one of the best.

Once you have given the metal a final clean don't hang about before giving it the first coat of etch primer. Aluminium oxidises very quickly. Even though you cannot see it.

Nick

[Robin GrahamParticipant @robingraham42208]

Thanks Nick – seems Cromwell supply the stuff, there is a branch not too far away from me. £'s not a problem if it actually works

Rob.

[Nick_GParticipant @nick_g]

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I don't know this, but I would strongly imagine that there will be youtube videos about how to apply it best.

Nick

[Joseph Noci 1Participant @josephnoci1]

If that etch-in-a-can works, then that is a neat and 'clean' method for sure. But, I have yet to find such a solution actually providing a durable surface that has properly adhered. All the spray can 'solutions' and even the chromate etch primers that you can spray on or paint on with a brush, have solvents that evaporate very quickly, giving very little time for the etch to take place. Even industrially it is recommended that the aluminium is first etched, and phosphoric acid is the recommended method, and then rinsed in clean water, dried, and then painted with an etch primer.

If you do not wish to pre acid-etch, a possible viable primer is a two part epoxy etch primer – the rate of cure is much slower than the fast dry spray can stuff, and the etch has some time to occur – BUT the aluminium must be scrupulously cleaned first! No oil, finger prints, etc.

If you dig through my numerous posts and photos of my machine refurbishments you will see I ended up painting many aluminium parts, panels, etc, and these have to stand up to lathe and mill swarf, coolant, cleaning fluids, etc – and they do! These were all acid-etched first before priming and painting, after trying many other of-the-shelf 'solutions' that simply did not work.

If your application is decorative rather than hands-on functional, that spray can of stuff may just do the job!

The Phosphoric acid I use is actually a drain cleaner from the local hardware store – A 30% acid solution, and I put some in a plastic spray bottle – the kind you use to mist plants leaves, or spray liquid insecticides or whatever, and spray the metal surface till wet. Don't spray with too fine a mist setting – rather have large droplets , else the mist goes everywhere and burns your skin and clothes…

I first clean the metal with a liquid soap, a sponge, and water. Rinse, let dry a little, and then spray the acid on. The surface goes a milky white – leave for about 5 minutes – if any surface still shows shiny, add more acid – if the shine persists, then that area is oily so scrub it with a pot scourer for a few seconds. When all milky, rinse with clean water after 5 minutes, and dry – compressed air works well, or with lint free towels. Now an acid etch primer is good..PRIME RIGHT AWAY!

Use nitrile, or latex gloves – I use Housewives dishwashing gloves, the type that goes almost up to the elbow.

If the job is small, you can just dip the part in the acid container, keeping mess to zero, and no spray mist.

If parts are large – DO THIS OUTSIDE, unless you want the rest of the workshop to rust from the deposited acid mist and wear goggles or a face helmet, like those used by Grinders..

Here's where I did the acid etching…

Joe

And the Painted Parts..

[john carruthersParticipant @johncarruthers46255]

I have this problem with ally telescope tubes, have you considered anodising or powder coating?

[Clive HartlandParticipant @clivehartland94829]

Anodising is both durable and saves a lot of faffing about. Choose AA25 for heavy duty and often handled items.You need to make allowances in machining to keep finished sizes within tolerance, plenty of details on the internet for tolerances.Less used items you cans specify AA12 or AA15 finish.

For coloured items supply a sample as the Technician will want to see how colours take to the metal you used.

I have tried a Black spray for baking on and it was useless as after few sessions handling it it just wears away. I did not etch first though.

Clive

[jann westParticipant @jannwest71382]

one suggestion I see in marine circles, after first having cleaned and prepped (but not acid etched), is to put on the first coat of paint and then lightly sand it. The logic is that you sand away the oxide into suspension with the paint, the paint then dries whilst providing a barrier to the formation of a new oxide layer.

[Ian S CParticipant @iansc]

Zinc Chromate primer then two pot epoxy top coat was our most durable paint sceme for agricultural aircraft, mind you that was 50 years ago. This Cessna 185 was a topdressing machine owned by Rural Aviation the topdressing branch of the firm I worked for, and rebuilt for the Gore Aero Club in Southland NZ.

These two photos about a week apart.

Ian S C

Edited By Ian S C on 02/10/2017 09:57:23

Edited By Ian S C on 02/10/2017 09:59:49

[mark smith 20Participant @marksmith20]

I bought a vintage 1940s Hartleys Botanic  aluminium greenhouse (10` x 16`) for £50, the frame was all chalky and discoloured , i used a two pack chrome based acid etch primer straight over the chalky surface ,followed by 2 pk epoxy and 2pk polyurethane . This was 10 years ago, the paint still looks like when i put it on . No sign of the paint film lifting or chipping at all. (hell of a fiddly and tedious job to do though).

I believe you had to apply the following coat as soon as possible after the etch primer which dried pretty fast.

Edited By mark smith 20 on 02/10/2017 10:19:07

Edited By mark smith 20 on 02/10/2017 10:25:10

[RobboParticipant @robbo]

Another vote for Nick G's U-pol acid etch #8. Have used this with good results. Checked it was recommended ( on petrolhead forums) beforehand. Bought it from Halfords.

[Philip RoweParticipant @philiprowe13116]

Sadly my experience of Upol does not match with others. I used it on an aluminium bridge for my garden railway as recommended by the supplier of the bridge. Within 4 days it was flaking off in quite large chunks. Now I will admit that my preparation was not as good as it could have been, you can imagine all the little nooks and crannies on a fairly accurate to scale girder bridge, so my preparation consisted of a rub with plastic wire wool (sorry I can't remember the proper name for it) followed by careful wiping with alcohol wipes with the acid etch applied soon after. You can imagine my dismay when it lifted so soon after, of course there were areas where the paint had stuck like the proverbial.

My solution was to get the entire bridge 1.4mtrs long bead blasted by a local firm which removed all traces of the paint and left a superb dull grey finish. I had originally intended to paint it but in fact I never have as I do like the finish left by the bead blasting and this has been outside in all weathers for 4 years and still looks good.

As far as I am aware the media that the blasters used was in fact walnut shells, whether this has any effect on the durability of the finish I don't know but at least it's better than the painted finish.

Phil

[Ron ColvinParticipant @roncolvin83430]

The question that comes to mind regarding the failure of paint on a oxidised aluminium surface, is does the paint peel from the oxide?, or does the oxide layer peel removing the paint with it?.

[Phil WhitleyParticipant @philwhitley94135]

The problem with aluminium is that the oxide forms instantly on contact with air, so all the prep is for naught, apart from degreasing down to the fingermark level, which is a must. The idea of a spray on etch primer is that it eats up the oxide and seals out the air at the same time so that re oxidation cannot occur. I used a yellow zinc chromate etch primer on landrover body repairs, and rarely had a problem, but it must not be applied too thickly. The finished panel should look as if it has been sprayed with a yellow tinted laquer that you can still see the metal through, and not sanded , but buried in several coats of primer before any flatting takes place. If you are doing a lot, buy a litre rather than aerosols, MUCH cheaper. As Clive says, depending on what the component is, anodising could well be the way to go.

[SamsarandaParticipant @samsaranda]

Yellow chromate etch primer was always used on aluminium components and aircraft when I was in the Air Force and the finish topcoat that was applied over it always stayed on. Aircraft finishes take a lot of stick both on the ground and in the air. I would use a yellow chromate etch.

Dave

[Robin GrahamParticipant @robingraham42208]

Thanks for further replies. Concensus seems to be that for a professional and lasting finish the spray-on primer is probably not going to deliver. However, the parts I want to paint (rails on which to mount lighting for a wheelchair) aren't going to be subjected to much more than an occasional rainshower, so I'm going to have a go with the U-Pol – I can well imagine the failure of Philip's bridge painting project using the stuff was due to the difficulty of preparing a complex structure like that properly. It's for sure that once a chink develops and water gets in the ali will oxidise and the new oxide layer will lift the finish. I've seen that on powder coated garden furniture.

In reply to Ron's question about the mechanism of failure, I don't know for sure , but my chemical instincts tell me that the paint parts from the oxide layer rather than the oxide layer parting from the metal. There must be a strong bond between the Al atoms on the surface of the metal and the oxygen atoms in the oxide coating or they wouldn't be so fond of each other to the exclusion of other attractions.

Anodising has been mentioned by a couple of people and that's probably the best way to go. I'll experiment with that myself, but can't afford to take the parts to a professional outfit – the work is pro bono and there's a limit on what I'll spend.

Thanks again for suggestions, Rob.

[Perko7Participant @perko7]

I've noticed that a lot of panels on cars these days are aluminium, might be worth having a chat to the local panel-beater to see what they use??

[Ian S CParticipant @iansc]

The panel-beater will probably just fit a new panel from the factory, these alloy car panels are rather complex.

For small items anodizing is the way to go, put anodizing in the Keyword box, there are some good threads on the subject, some of the forum members have had some good results DIY.

Not under anodizing, but someone, maybe JasonB will know where its hiding

Ian S C

Edited By Ian S C on 03/10/2017 10:30:22

[the artfull-codgerParticipant @theartfull-codger]

I've used ici long life 2 part etch primer on all my alloy castings & have done for years, out of the molding box fettled & a light wire brushing then hand paint, never had any peeling, I usually spray with 2 coats enamel paint after, I've just bought a litre as I used my last up [well you've got 2 litres as you use a 50/50 hardener mix] but if I was to buy any more it would probably be 2 pack epoxy primer, as it's non porous.

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