Aircraft Painting

[Richard Moore]

How many litres of top coat (grey over grey primer) would it take to spray a Cub?

[Brian Hope]

That’s a bit of a ‘how long is a piece of string question’ I’m afraid. How good you are at spraying and what system you use will make a difference. Ball park figures in the Poly-Fiber manual suggest 10 US gallons of their Poly-tone, which is their traditional ‘dope’ type of finish. 6 US gallons if you use Aero-thane, which is their polyurethane two pack high gloss system, and 4 US gallons with their sister company’s Ranthane, which is also a polyurethane type material. Bear in mind the two pack poly systems require hardener as well as thinne, and are more expensive per gallon, so the reduction in material quantities does not necessarily mean a reduction in cost. Polyurethanes are are more hazardous to health than dope and definitely require air supplied masks. They are also less forgiving materials to apply.
You will save material if you use an HVLP system, so maybe you can reduce the quantities a little there.

[Terry Lee]

Not disputing your figures, Brian, re the 10 US gals of Polytone to spray paint a Cub, I've read that myself in the Poly-Fibre manual, but it does seem like a hell of a lot to me. Would that provide 2 or more coats?

[Richard Moore]

I asked Stewart Systems, who make a two pack water borne polyurethane, "how much paint will I need for an aeroplane slightly smaller than a Cub(Groppo Trail), plus a bit to practice". They suggested three and a half gallons. Thirteen and a bit litres seemed an awful lot, it's over twice the amount I'd use if rolling it (not going to). The RAL colour I want will have to mixed to order, so ideally a single batch, which is why I'm trying to determine "how much".
Somewhere I read that the air temperature in an HVLP system can cause problems when using water borne paint, and as I have a compressor that can supply the cfm for the Iwata gun I have in mind, I'd dismissed HVLP; but I'll have another look.

[Brian Hope]

I agree ten US gallons of butyrate colour dope sounds a lot but you will be putting three cross-coats, that's six coats, of it onto the fabric. You spray on a coat and then go over that again with the second coat at 90 degrees to the first coat - that counts as one cross coat. You then do a second cross coat and when that is dry, flat it off with wet and dry, and then do a final cross coat.

With polyurethane a typical technique is a light, almost dust coat, then when that has tacked off, a full coat followed by a second full coat. Needless to say different manufacturers recommend different techniques, and different painters also have their own ideas, there is a degree of self-expression with painting. Polyurethanes cover much better than butyrate dopes, which is why you need fewer coats. Dopes are low in pigment content and it takes more coats before you get the full colour. Some colours are worse than others, reds in particular need more coats.

If we are talking about a Trail (all aluminium) rather than a Cub (fabric), then you don't want to be using dope (Poly-tone) anyway. Dope is not really ideal for other than fabric as the adhesion can be pretty marginal, particularly on something like fibreglass or aluminium. Use either an automotive cellulose or a two pack polyurethane. My personal view is that aviation specific paints are not necessary for aluminium or composites, but are absolutely necessary for fabric. Plasticizers added to auto paint and used on fabric have a limited life and long term the paint will harden and crack. If you get much more than five years before that happens you’re doing well; so if the aeroplane’s a keeper then do it right, you’ll be glad you spent the extra money.
My only caveat on that is if you are using an experimental roll on system using emulsion house paint. This is an interesting cheapo fabric painting idea that has been around, mainly in the US, for some years and is being experimented with now in the UK. I haven't tried it but I do think with the right paint and rollers, and a practiced technique, you should be able to get a pretty decent finish. It will not compare with a good spray job though. Likewise the roll on paint that Sprite sells for the Trail. You pay your money and take your choice as they say.

HVLP has all but taken over in the automotive industry where H&S and environmental issues have forced the industry clean up its act. They use water-based/borne polyurethanes now which need to be force dried (heat and airflow in a proper spray booth). The Stewart system looks very interesting and is now available in Europe via a chap in France. He has set up a covering/schooling type workshop south of Paris, I can dig out his details if anybody is interested. It is as Richard says, a water borne product and I wouldn’t mind seeing how well it sprays/dries, but I have never used it so cannot comment on any issues it may present to HVLP equipment. To be honest I’d be surprised if it was a problem with HVLP equipment. The kit I have is only a relatively cheap unit, around £300 I think, but it does a perfectly good job. I used to spray cars and lorries for a living, a long time ago admittedly and using what was then the norm - high pressure spray equipment. Having used HVLP I wouldn't go back to a high pressure system, it is so much cleaner with considerably less overspray, and if you have a compressor anyway, you can use that to supply an air fed mask instead.

[Ian Melville]

The Stewart system looks very interesting and is now available in Europe via a chap in France.

I've been buying mine through Airworld UK. Just cleaner, etch and Ekopoxy primer at the moment, but I intend to use their system throughout. Yet to use it due to touch up gun failing

Wish they would supply MSDS sheets though

[Brian Hope]

Hi Ian, the Stewart Systems manual says that Material Safety Data Sheets (MSDS) are available on request. Their website is at http://www.stewartsystems.aero and it has a 'contact us' facility.

[Ian Melville]

None of the product instructions mention MSDS, nor does the website. I wasn't immediately aware of a manual, so checked again to find the only manual is the fabric covering one, which I dismissed as my aircraft is metal. I downloaded and read that just now and it does indeed invite you to request MSDS. The manual is badly named as it also covers metal (steel/alu) and glass fibre structures.

Thanks for making me look again as there is some other useful info there.

[Richard Moore]

Had another look at HVLP. The first question from the man in the spray gun shop was "what is the free air cfm of your compressor?" All their HVLP guns need a minimum of 9 "free air" cfm; a lot more than a comparable conventional gun. That meant I'd need another, bigger, compressor with 13.5 to 14 "displacement" cfm. It was suggested, for a one off, to use a conventional gun.
Dan, at Stewart Systems, suggested using a modern turbine HVLP system. Don't have to deal with water or oil in the air and the higher temperature is not an issue because the paint has a long pot life. He advised putting couple of coils of the hose in a bucket of water to drop the temperature "if it makes you feel better".
A Fuji Mini-Mite 4 is now on order.

[Brian Hope]

Hi Richard, yes I should have mentioned that an HVLP gun needs a high capacity compressor, certainly larger than most amateur painters or hobbyists will have. The turbine systems are the best bet and that is what I have. I think I mentioned earlier that the bonus is that it frees up your too small compressor for running an air fed mask.

[Terry Lee]

Have you looked at LVLP guns, there are a good range to suit most pockets. Typically they only require 3 - 5 cfm and 25 - 40 psi, just a thought!

[tnowak]

Seen some info on another USA website about fuel issues and the Stewart system, which I thought worth posting here. Appears that some fuel leaked onto fairly new fabric, causing major issues.

Extract below (with minor edits):

"This is just a comment for information to those doing fabric work. I thought I would be starting up my aircraft this week but instead it is coming back apart. I just got it assembled and now a fuel leak has destroyed the belly. Yes I should have found the leak at the fuel valve but that isn't the big problem. About a pint to one quart,(at the most), of gas leaked, soaking the insulation in the left side and belly some running down the gear leg. The Stewart ekopoly peeled loose and the glue joints all came apart as well,(stewart ekobond), the glue/paint had been on about six months. I know my fault with the leak, but, I have never had damage like this over a 14 hour period, with dope or enamel before. I thought I would try something new with the waterborne urethane and sure wish I had stayed with Randolph products that I understand. My point is if you want to try this stuff I recommend starting with some test pieces to play with and see how it holds up for you before doing your whole project. I am not saying that I did not do something wrong in the application. I don't think so but even if I had the glue for instance is applied straight from the bottle and it melted in the gasoline. I have never seen Super Seam melt from an overnight gas leak. Same with the paint peeling off I've seen lots of fuel stains on dope and enamel finishes but not lifting and peeling from a fuel leak".

Tony Nowak

[G.Dawes]

I did my Jodel with a polyurethane finish and wished I hadn't because a little bit of damage mad it VERY difficult to repair the fabric. Straight forward dope finish would have been so easy to repair. Poly might look shiny but it is heavy and does show every little fault in the finishing. Never again.

[Brian Hope]

Interesting story Tony. I'm going to the RSA Rally tomorrow and if Frank, the French agent for Stewart Systems, is there I'll ask him about this problem.
Any deep gloss finish is going to show up imperfections more than a semi-gloss dope finish, that's why the secret of a good paint job is the preparation. Not so sure the poly finish is appreciably heavier, you use less coats of top coat - typically one very light and two full coats - compared to a dope finish. The paint job on a typical two seater is reckoned to weigh about 7-10kg so the varience in a dope or poly finish isn't going to amount to a great deal. Spot repairs on traditional poly coverings are very difficult but I believe some of the modern systems are better.

[tnowak]

Further update on the Stewart system paint issue.
Seems the problem may be due additives in some US refined mogas, but info slightly confusing.
Aircraft owner was using something called "Marathon Recreational, ethanol free, premium" autogas.
Other posts indicated that regular avgas hasn't caused similar issues for another owner.
I wonder if the additives used in US will eventually find their way over here?

Tony Nowak