Light Aircraft Association

Could anyone suggest a method of finishing or painting small steel fittings and recommend suitable products and suppliers? I've searched LAA/EAA archives on the web for ideas and had no success so far.

I have in mind using an aerosol etch primer first followed by a brushable epoxy paint. Or, perhaps, an epoxy primer followed by a top coat of some sort. Powder coating seems to be out of favour now.

So far, I have thoroughly degreased the bare steel parts & then used zinc chromate aerosols or "1 pack" aerosols of etch primer. The etch primer seems to do a much better job.

I'm looking for a seriously weather and "ding" proof system this time, since the parts concerned - the landing gear and assorted fittings -will dangle outside!

Hi Ian
Part prepared as required then 2K etch coat followed by 2 k primer and a 2k acrylic(or polyurethane) for me every time.
In my limited experience there are no shortcuts with finishing work.

Most of the German glider manufactuers use this system on metal parts like u/c frames and it seems to be a pretty much bombproof finish for utility working club aircraft.

Hope this helps

Cheers
Adrian

Many thanks Adrian,

That sounds good enough for me and I'll have a go!

Ian

Hi Ian,
I am not sure of the meaning of "2K", but Adrian's system description is the good one:
-one primer for bonding to steel or aluminium
-one anti corrosive paint
-one finish coat.
I work in shibuilding, paint is a major issue for us. I would add that surface prep is of capital importance, the best is to sandblast the parts. You will get rid of everything compromising the paint to stick to the part, and would create a profile (rugosity) improving the sticking. On aluminium, we allow in the yard maximum 24 hours between the blasting and the etch primer, to avoid aluminium micro oxydation leading to blistering in a few months (within the warranty period, indeed ) The best is to blast, blow and paint the etch primer in a raw.

The primer and finish can be one coat, the AC MUST be two coats: the two coats technique is the best one to make sure the film is sealed (epoxy systems work on that principle, create a 100% proof barrier to protect the parts). In fact, even in a paint cabin, there is a probability to get dust or foreign object in the paint film, for each film. By applying two layers, however, the probability to get the two films polluted at one exact peculiar location is nil.

One other important matter is film thickness. Do not make paint to thick, it will just make the film brittle. If you paint with brush or roller, the DFT (dry film thickness) is much lower (10 to 50 µm) than with an airgun (30 to 200, more with airless pumps). Depending on the brand you use, try to be within the thickness prescription of the manufacturer, given in the technical datasheet. You will get also the information on minimum application temp and overlap regarding temps. Those are important data, take them in consideration. I have been using two componants (2K???)epoxy AC and two componants PU finish (UV protection of the epoxy) for 15 years now with great results.

Hope this will help,

Bertrand

Bertrand,

Many thanks and I am very grateful for your advice.

I don't possess a decent spray gun or the necessary breathing gear to allow me to attempt to safely spray the exotic paints that are needed, so I will have to use a brush.

I can't afford to have too thick a layer of paint, since the clearances in most of the fittings are rather small and cannot accommodate a thick layer of paint. So using a brush would seem better for me, anyway.

Ian

Hi Ian,

you can achieve great results with brush and roller, cross your mouvements while painting.

Bertrand

bertdeleporte, thanks for your forum replies, can you say what paint products you use - maker /type from where? I am interested in painting steel and also epoxy. Also some cockpit paint. I cant see why I should need to order paint, maybe made by dupont form the US.

Hello Graeme,

I presently work in a shipbuilding yard. We work with several brands, mainly International, Hempel, Jotun. I also worked a few years for a truck bodybuilder: other market, other brands (Glazurit from BASF, Autocolor by PPG, Omron by Dupont), same products: zinc oxyde primers, epoxy AC, PU finishes...

I thus recommand, according to your habits/residence, to go to your shipshandler/auto part supplier. They will both have catalogs/ref lists for products, with technical data sheets and application recommandation. Stick to the guidelines, you are bound to succeed!!!

Once again, for painting, do not make shortcuts, the better you prepare the parts, the best will be the finish.

Bertrand

In a very old PFA magazine it stated that the frame tubing in older aircraft were flushed through with hot linseed oil after manufacture to protect them internally. What brands of etch primer are there for chrome/moly steel ?

Hi Bill,

zinc primer applies for all steel alloys. For the technique you are speaking about, it still exists in shipbuilding for narrow voids and cofferdams where acces is not possible for blasters. The capacity is pressure tested, then filled and emptied with, depending on the yard, specific protective oil, plain mineral or animal oil, floated ox fat...

The idea is to protect as far as reasonnable with a very flexible coating. The theory asks for a pressure test to make sure the capacity is tight; the oxygen trapped inside the capacity would "burn" and turn into oxide a little bit of steel, and then the process would naturally stop, providing the capacity is 100% tight and no oxygen (air or water) can feed the process.

I have been told the tube framing of Aerospatiale Alouette helicopters was lushed and filled with slightly positive pressure nitrogen (no moisture in pure gas), a pressure gauge being visible inside the cockpit. A pressure drop would then indicate a fracture in one of the welds. I could never check by myself if it was true, but the idea sounded clever to me.

Bertrand