Warbird Information Exchange

The recent thread about the particulars of airplane polishing got me wondering about airplane paint.

If you had a steel tube fuselage, such as a T-6 or BT-13 center section, how would you finish it? Sandblast, etch primer, epoxy? Zinc Chromate? Rustoleum spray paint from a can? Ruddy brown automotive primer? PPG auto paint? Testors model paint in large quantities? Latex house paint? Powder coating instead of paint?

Just curious.

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Brad

Hi Brad,
Interesting post mate, and I'll provide my thoughts. Before I go any further, my project is a CAC Wirraway, which has a steel tube forward fuselage frame (or centre section as you called it) very much the same as the T-6. I'm not far off having to deal with exactly what you're asking about, so I've already put some consideration into it.

Ok, my plan of attack is the following. Subsequent to all repairs that need to be made to the structure, the frame will be either sand or bead blasted. Probably bead-blasted with glass beads (AB grade Ballotini the likely specimen). Don't use garnet beads. As soon as possible (meaning a matter of hours, not days or weeks) after blasting, the frame/s will need to be primed. Personally, I'm probably going to use Zinc Chromate as per original. Many people knock it, I've researched into it, and haven't really found anything against it. It's apparently somewhat dangerous to your health, so if you don't want to use Zinc Chromate (or if you can't get it in your area), then Zinc Oxide is available-which is supposed to be safer.

My top coat is tentatively planned to be Nitrocellulose lacquer, although this is still under investigation. Once again, many people will knock the use of lacquer in favour of some of the great epoxy products now available. Both have distinct advantages and disadvantages, and there is likely not to be one definitive 'perfect' product to use. It is important however to ensure that the surface finish is not just left as being exposed Zinc Chromate. If you want the Zinc Chromate colour, then get a proper top coat mixed to the desired colour of Zinc Chromate. Also, don't even consider having the frame powder coated! Unfortunately I don't know enough about the specific details of powder coating, however it can have adverse affects on the underlying material. When considering such a structural element as an aircraft frame, it's best to avoid it.

As mentioned earlier, these are just my thoughts. Many others would have differing opinions. Hope it might be of some interest to you, and if I can help out any further, please feel free to contact me either through here or privately. All the best.

Cheers,
Matt

Excellent info Matt! I have heard the exact same warnings about powder coating, and the need to top coat your Zinc Chromate (if you were going to leave your tube frame that color).

Brad, as you know we are doing a Fairchild PT and the tubing will be Zinc Chromated, but then top coated with silver per Fairchild factory procedures. But the part that I wanted to add here is, don't forget to take care of the inside of your Rawdon tubing with some sort of a tube seal, (Stearman, Fairchild, and others used Linseed Oil although there are other products out there now). I have been told the procedure should be to bead blast, prime, and paint the tube frame, and THEN do the tube seal, because if you get the Linseed Oil (for instance) on the bare tubing, the primer/paint will not stick to the frame.

Make sure you let us know how it goes.

Steve

P.S. Don't forget, we've got that Rawdon canopy frame here for you.......

What are the concerns covering powder coating? I hadn't heard of any. I would think that maintenance would be a big advantage for powder coating.

Patrick

Hi All,

Concerning Steve's comments concerning taking care of the inside of your tubing, is anyone familiar with any restorers of 4130-tubed warbirds pressurizing their tubing with nitrogen? I haven't done this personally, but I've heard that it's an effective way to check for pinholes in tubing and welds; and it would be nigh impossible for corrosion to start if you had replaced all the oxygen inside with nitrogen.

Of course, this would take all the fun out of dribbling hot linseed oil up and down your fuselage--and killing all the grass when it misses the drain bucket on the way out!

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It was a good idea, it just didn't work.

Patrick,

First and foremost, let me preface this comment with the statement that I am NOT an authority on this subject, there are many of you on here that are much more knowledgeable than I on stuff like this, (Brandon, where are you?), but here is what I can remember about powder coating. (And it aint much!)

I think I remember this topic on the Stearman Restorer's Association forum board, and I guess I will have to do a search to see if I can find it. But if I recall correctly, one of the reasons behind the warning not to do it, was that the coating provides a hard shell that doesn't always stick to the surface, and this allows corrosion to get in and spread under the shell undetected during inspections. Again, I do not know this from experience, just warnings from other restorers.

It is interesting though to see that many kit built experimental aircraft (like the Kitfox, for example) come with the frame powder coated from the factory. (Or at least they did when I looked at the Kitfox last). As you say Patrick, it is a very tempting option because of it being so durable.

Now, over to someone who REALLY knows what they are talking about.....

Steve

Powder coating will not hurt steel. The baking temp is high for aluminum and can warp thin parts. The duration is short enough that the parts won't be annealed though. I personally wouldn't use it for any structural aluminum components.

I had my Stearman engine mount powder coated, and here are my observations....

Powder coaters usually don't have much experience with tubing clusters, so the coating may be thin or missing in these areas and will rust later on.

The coating is much thicker than paint, so machined tubing clamps may no loner fit properly. You may need to drill the coating out of any bolt holes or chase any threaded holes.

A friend of mine had his T-6 frame powder coated. They had to do it twice to get it all covered, but it turned out well. 8 years later he is quite happy with it still.

As others have noted, traditional zinc chromate primer does not seal the metal and will surface rust later on. Also, on fabric covered aircraft, fabric adhesives and dopes will soften some primers due to the solvents- which could cause your fabric to not adhere properly to the tubing. You might as well just go with epoxy primer which is a good one-shot coating (recommended by Stits), but color matching can be problematic so you may end up with a top coat anyhow.

Sandblasters generally don't do a lot of aircraft work, so watch them carefully. They go the easy route, which means very coarse media and high pressure which may leave a rougher surface finish than you desire. Have them use a finer media, or go to a place that media blasts cars that understands how not to use high pressures and coarse media.

Use caution with painting right over sandblasted tubing. The media could be contaminated. I'll get chastised for this, but I recommend treating the bare steel before painting. You'll probably be OK, but what if you were the one in a thousand that had a contaminated surface which caused the paint to start flaking off of your T-6 frame a week before you were ready for your first flight after restoration? Etching sounds like cheap insurance to me...

Personally I'm partial to the epoxies for the finish coat. I've seen a lot of peeling Imron, but that is probably due to poor surface prep.

Dan,

(Boy, I just can't shut my yap today!)

I haven't heard of anyone doing this with Warbirds, but I know that some aerobatic guys do this with their aircraft and leave a little pressure in the tubing with a pressure guage attached and this way they know if they've cracked any of the welds in their manuevers by looking for a pressure loss on the guage after each flight.

See ya at Oshkosh my friend!

Steve

Zlin 526's have this in the main spar carry through structure in the fuselage. It may be common in some of the other eastern bloc aircraft as well.

HI IVE HAD LOTS OF EXPREIENCE WITH TUBE A/C,WE MEDIA BLAST THEM,EPOXY PRIME AND PAINT FINAL COLOR.THE T-6 BT-13 STEARMANS AND THE PT SERIES WERE ALL WHITE CAD PLATED (DULL SILVER/RUSTOLIUM CHAIN LINK FENCE PAINT IS PERFECT!!!)WEVE TRIED THE POWDER COATING AND ITS WAY OVERATED!! IT CHIPS EASILY,ITS VERY HARD TO GET INTO THE CLUSTERS WHICH IS A MAJOR CONCERN,AND IT DOES ADD A THICKNESS TO ALL SURFACES.ALSO IS LIMITED TO THE COLOR SELECTION.EXTERIOR IS ALWAYS A PERSONAL CHOICE EPOXYS ARE GREAT BUT IF YOU WANT A FLAT FINISH ADD THE SAME AMOUNT OF FLATENER AS COLOR 1 TO 1 MIX.IT WILL COME OUT PERFECT!!FOR SILVER FINISH IF YOU LIKE THE SHINNY STUFF GO WITH EPOXY IF YOU WANT IT TO LOOK LIKE WW2 USE THE RUSTOLIUM CHAIN LINK FENCE PAINT (TOO SEE A EXAMPLE SEE DICK KNISELYS T-6 IT WAS PAINTED ALMOST 20 YEARS AGO AND STILL LOOKS GREAT!!THE NICE PART IS YOU CAN TOUCH IT UP WITH A SPRAY BOMB AND BLENDS IN PERFECT!!PLUS IT STICKS LIKE GLUE!! WHEN PAINTING FABRIC PUT LOTS OF FLEX AGENT IN PAINT!! I DOES WORK!!IF YOU WANT A ZINK CROMATE FINISH INSIDE PRIME AND PAINT WITH AN EPOXY MIXED TO THE SHADE YOU LIKE ADD LOTS OF FLATENER AND IT WILL LAST FOREVER ALMOST.ALL ALUMINUM SUFACES MUST BE SCUFFED WELL FOR THE PAINT TO STICK SCOTCH BRIGHT AND ELBOW GREASE!!WIPE ALL METAL PARTS WITH MEK AND SPRAY.GOOD LUCK!!MIKE