Paints and Primer questions and advise for 4130 steel

[animal]

OK as some may know I am in the process of restoring a classic Helicom Commuter Jr. helicopter. the airframe is 4130 steel covered in fabric.

my question is what is the best type of primer and paint to use on the 4130 steel that will last.

I have been told to go with an epoxy primer, but I don't know any thing about them.

So has any one used the Epoxy primers? Pros and Cons?

I know there are some good painter on here.

will be sand blasting the whole airframe next week sometime.

right now the airframe has about 3-4 layers of paint on it with lots of hanger rash and scratches.

plan is to sand blast down to the bare metal, inspect all the welds and tubes then repaint with a tough primer and paint.

Thanks in advance for the advise.

[Vance]

Hello Timothy,

Most of my racing bike frames were built from 4130. We would strip them down to bare metal each year to look for cracks.

We found that if we sandblasted the metal it tended to hide the cracks so we always did it by hand with paint stripper.

We used paint that would show the cracks and stayed away from the epoxy paints and powder coating because the cracks did not show through as soon.

Most cracks would appear near a weld so that is where we focused looking for cracks.

If it was my project, given the age and state of your aircraft and its history I would strip it down with paint remover and go over each weld very carefully looking for cracks. I would also look very carefully for evidence of corrosion. Sand blasting tends to mask the evidence of these things.

We also had an air fitting on the frame so we could pressurize it. Even a small crack would cause an overnight leak down and small cracks are the beginning of big cracks.

We used normal primer and lacquer because it was easy to touch up when we made changes and we were going to strip it the next year and it was easy to remove.

In my opinion the fabric should be painted with a paint specifically formulated for fabric with the correct layers based on what process you are using. Enamel is often used for a top coat and in my experience it doesn’t hold up well.

Thank you, Vance

[Earthboundmisft]

Check with the fabric manu. for glue compatible coatings. Poly Fibre is the best in my opinion, and their cement will 'lift' anything but an epoxy. I have covered a Pitts, and two Skybolts in the last 8 years, using the Poly Fibre system. Works great.

[scottessex]

Vance has a good point. I like epoxy primers, but I have not used any on aircraft.
The disadvantage of a lacquer is that it is not very fuel resistant. Plus Lacquers are hard to get in some states since they have been outlawed by the EPA for VOC emissions.

If you decide to go with epoxy I would use PPG DP series primer, there is a gray/green (DP40), black (DP90), and white (DP 48) . It is dang near bullet proof. Then use a single stage poly for gloss and chemical resistance. That is just my $0.02 since you asked. :)

The DP primers are about $75qt.
Or check out this fine line of products, I used them on my wife's mustang.
TPC Global Kustom
shop (http://www.tcpglobal.com/kustomshop/kspepoprime.aspx)

Good paint and I saved a bunch of money.

[brett s]

I'm with Vance, very good advice there.

After chemically stripping, look for tiny pinholes in the tubes especially on the lower sides as they'll rust from the inside out if not properly protected - by the time you see that, the tube is toast.

[animal]

Ok thanks for the replys, every bit helps.

earthbound ,brings up a good point about the cement. so that is why I am looking at a good epoxy paint primer. since I plan to go back with the colors it was when built ( blue cab, white frame ) the white epoxy primer sounds like the best choice. now to see if i can find it local or if I will have to order it online.

thanks for the Link Scott.

so what I am thinking now it it may be a good idea to strip the paint, then after inspection maybe sand blast it and primer it.

Oh well, no one said doing it right was easy.

[Arnie Madsen]

I would not sandblast it at all Tim. It will distort the metal. Ask anyone who has sandblasted a car door or hood etc. It is ruined. You can experiment with a scrap piece of light wall 4130 tube. Sandblast one side of it and you will see it is no longer straight. Stay away from sand except for heavy industrial equipment and gravel truck boxes etc.

If you are determined to blast , use a soda blaster. Same principle but uses baking soda instead of silica sand. Very safe. I think Justin Travis did his Hughes 500 with Soda. Does a very nice job. Rates are around $200 an hour , and a lot can be done in an hour.

Good old paint stripper is still the way most guys do it.

[HobbyCAD]

Hi There all tuned into this thread,

My cent's worth of info is, there are some definate do's and don'ts when using abrasives in the aircraft structure environment.

Some abrasive materials, because of it's chemical composition, can cause corrosion on the material you are working with. Some metal oxides used as an abrasive will react with the aircraft structure metal. Science 101 tells us any dissimilar metal in contact with each other causes a reaction. Make sure your abrasive is compatible with the material you are using it on. Abrasive safe for steel, might not be safe for aluminium. Thus, when working with steel and aluminium in the same workshop, have 2 clearly marked set of tools, seperate grinders, seperate sanding belts. When using the aluminium tools on steel, you will deposit the aluminium onto the steel, or vice versa, causing an electro chemical reaction. Use the steel working tools on steel, the aluminium on the aluminium. It means one side of the grinder for steel, the other for aluminium, 2 sets of files, 2 sets of sanders, 2 sets of cut-off tools. Especially the cut-off tools, you don't want to contaminate a new weld area.

Correct abrasive for steel might be silicon carbide, for aluminium might be aluminium oxide. Best is, go for a synthetic abrasive media, such as Scotchbrite pads.

Abrasive blasting is the best way to remove corrosion, followed by polishing with a fine synthetic media, to prevent the loss of fatigue strength. Again, use a non metalised fine media such as silica sand or glass beads. Remember, don't use the same "used" abrasive from steel onto aluminium, seperate the 2 bags, if you are going to reuse it again. Watch out for immediate rusting of steel components, after media blasting. Blast on a dry day, with air from a compressor with a good water trap. Blast a little, then do corrosion protection, and so you carry on. Don't touch the frame with your bare hands, instant grease or moisture contamination, use gloves.

For corrosion protection and priming, don't use a self-etching primer, it will etch the outer layer, makes it more brittle, causing possible micro cracks, that turn into eyball popping cracks. Same story as color anodising, remember the story of the prize-winning Safari with black anodised control tubes?

Don't use chemical corrosion removers, it also etches the top layer of the metal. The chemical reaction makes the surface brittle. Same for paint strippers, make sure it's not too "wild". I've used some products that were so caustic, it just about disolved my workpiece !! I remember a friend wanting to paintstrip the antifouling off his fibreglass boat, other than sanding it off, with the poisoneous dust flying around everywhere. Good grief, he slapped it on in heaps, just about covered the boat bottom, when he realised trouble was brooding. The paint stripper worked well, the antifouling scraped off with absolute ease, but then he noticed, the gelcoat was being scraped off as well. By the time he realised that, all the gelcoat was all soft, totally ruined his hull. It stripped down to bare fibreglass !! He should have used a stripper safe for gelcoat, not industrial strength "I'm in a hurry" stuff. Same for your fuselage, get the correct aviation strength stuff.

While you are blasting or cleaning, use a zinc chromate primer, as you go along, to cover up the work. It serves a dual purpose, it creates a sacraficial corrosion layer on the metal, it also serves as an ideal basecoat for the heavy duty primer you want to put on later, like the 2-pack stuff you guys are suggesting. The spraycan zinc chromate primer dries instantly, no need to wait while you blast, for the primer to dry. Spray it on lightly, almost only a fine misty layer. It's very compatible with most other painting or priming systems. May I suggest a product like Tempo Aviation Zinc Chromate Primer, comes in handy aerosol spraycans.

Just a final though, make sure your heavy duty primer or topcoat does not harden with age. Once flying again, your frame will flex, causing hairline cracks in hard paintwork, letting moisture seep in under the paint. A heaven for now unseen corrosion. So Tim, with your "senior" fuselage, it's a good idea to strip it down to bare metal.

These are my methods, for "gospel" facts, research latest issue of the FAA "bible" AC 43.13-1B, chapters 4 and 6, on metal structure inspection and repair. Tim, it's pure "elbow grease" from now on....., but the pickings are rich !!

Regards,

Francois

[Minnesota_Mike]

Those are some absolutely fantastic "Tips" there Francois!!!
(I liked what Vance had to say also...especially "Pressurizing" frames...never heard of that one).

I'm putting the contents of both posts in my Archives..!

Thank You guys for great tips and advice.

[HobbyCAD]

The French helicopter industry had started with pressurising their tubular steel airframes years ago. The old Sud Aviation Alo II springs to mind. They have a simple pressure inflator nipple, same as on nitrogen filled struts, a small pressure indicator in a visible location, and a pressure activated electical switch. During pre-flight, you look at the gauge to see if the pressure is in the green, much like a fire extinguisher indicator. If during flight you have a crack starting, the pressure escapes, and triggers a warning, be it an audible or visible warning in the cockpit. Such a simple low cost non-engineered thing to do, little holes drilled into the tubes before the are welded together, and 3 x 1/4-NPT threaded inserts put in somewhere. By pressurising your frame, you immediately see if your welds are sound, as well as a warning mechanism for the rest of the flying life. Why is it not stock on all machines??

Francois

[animal]

yeah I know a guy in Canada building a safari, and he has his tail boom pressurized. has a small gauge that he can view and a sender to a warning horn.

pretty smart set up.

[tyc]

I would not sandblast it at all Tim. It will distort the metal.
... If you ... blast , use a soda blaster. ... Does a very nice job. Rates are around $200 an hour , and a lot can be done in an hour.
Good old paint stripper is still the way ...


As far as I know and have learned over the years Madsen is right. Use paint stripper and once the paint is off, use a dye penatrant to look for fractures in the welds.

Once you're sure all is well, consider using Rustoloum Primer. Used to use it to paint the HY80 steel in our bildges and never had to worry about rust for years. All you need is the Rustoloum Primer. Don't waste your time painting over it, unless of course it has to be "pretty" but that will simply add weight and won't make it fly any better.

Hope this helps.

tyc