type of powder?

[PaulW]

Hi everyone,

I am building a kit helicopter with a chrome-moly tube frame. I want to powder coat it for a nice, durable finish. The problem is that this frame will be subject to vibration and cracks need to be spotted early. I have heard that powder coating finishes are sometimes too flexible and thus would not allow a crack in the steel to be visible through the powder coat, ie the powder coat would not crack even though the underlying steel did.

My question is - is there a powder coat which is much more brittle than the usual one which would crack along with the steel? This powder coat will NOT be subjected to weather, the frame is totally covered by body panels and this aircraft will not be flown in the rain or snow. Any advice would be helpful.

Thanks in advance,
Paul

[Fireblade]

Well, not sure about the coating being able to crack, unless ya do it wrong, making the powder brittle, but if ya do that, why do it? But, about that chrome-moly frame, did you buy it from someone? Do you know if the frame was heat treated to relieve stress from the welds? chrome-moly, is a strong, light weight material, but when welded it creates stress areas that need to be relieved by means of heat or cryogenics to ensure a long lasting final product. This is not done by many manufactures, ther reason?? I have no idea, it in my opinion is ridiculous to sell something that is GOING to crack. My advice to you, is search for someone on the internet that does cryogenic freezing or stress relieving, and get your frame done that way. Yes, all materials have a expiration date, but by doing this, it would extend that date dramatically for you. Stress relieving basically aligns all the moelecules within a substance so that everything is equal. Say you have a tic-tac-toe square, you put all the x's and o's in 3 boxes, now that is 9 total but only in 3 boxes. That is how something ends up when you weld on it, when you stress relieve it, they align thenselves to fit in all the boxes, not just some. Heating changes the moelecular structure from, I think it is call austenite, to another form of material. I forget the names off the top of my head. One thing, DO NOT HAVE SOMEONE HOLD A TORCH ON THE WELD AND MAKE IT GLOW LIKE A CHERRY THEN LET IT COOL!!!!! That is not stress relieving, that is called annealing, and when ya do that, you basically make the piece into a rubber band, losing it's strength. Why would ya want strong stiff chrome-moly held together with rubber bands It won't crack, but it sure as hell will bend all over the place. Powdercoat that frame after getting this done, and ya should be fine.

[rivermasternc]

Paul:

Yes, epoxies are marginally more brittle than polyesters and much more brittle than thermoplastics such as polyethylene, nylon, and fluoropolymers. For your application, a decorative epoxy, hybrid or polyester will do the trick. Decorative finishes may be applied in the 1.5-3.0 mil (0.0015"-0.0030") range of thickness. These thermoset decorative coatings will reflect the condition of their substrate if properly applied.

-Scott