Aluminum / Silver Colored Paint - Watch Out

[K1JJ]

Hola,

Back in the early 70's I built my first 2KV HV supply using old TV can capacitors in series. To hold and insulate the caps in place, I bored 1" holes in a 12" X 12" plywood board. To make it look purty, I spray painted the board silver.

 I fired up the supply and the caps started arcing to each other. Turns out that this silver colored paint was very conductive, even at DC. Evidently, the manufacturer added aluminum particles for the color.

Fast forward to 2008. I figured by now silver colored enamel paint didn't use this coloring gimmick any more. I enamel silver painted a small plywood shelf inside one of my HB linears that was a few inches above the tank circuit.   I even measured the board for DC continuity and it was over 2 megs - out of the multimeter's range.

The other day I fired up the linear on 75M and watched RF "sparklers" coming out of the plywood board.  It looked like tiny bits of metal catching on fire. The board started to burn before I could shut it off. This occured in the middle of the board away from any parts. The board was supported by metal rails that were grounded.  I replaced it with the old JJ Plexiglas standard and all is OK-fine.

Just wanted to say that in this day and age, SOME silver / aluminum colored paints may not conduct DC current, but sure come to life with 3.8mhz RF.  I can just imagine how they would act at 14 mhz and higher.

Has anyone else made this blunder or am I the only one?   

T

[k4kyv]

Read carefully the contents on the label.  You will see that it contains metallic powder of aluminium, zinc or whatever metal it's supposed to simulate.  I opened a can (non-spray) that had been sitting around for several years recently and the metallic solids had all settled to the bottom, with a clay like consistency.  It took a lot of stirring to get it properly mixed once again.

I have found aluminium paint to be a better rust preventative than the so-called "cold galvanising" stuff.  I painted the steel casting on my tower base insulator initially with Cold-Galv.  Within 3 months, I could see rusty spots showing through.  I repainted with Rustoleum aluminium paint, and after a decade it still hardly shows any sign of rust.  I also used it to touch up some of the damaged galvanising on my tower base section where copper had leached from the open wire feeders and caused corrosion.  That was almost 20 years ago and it still hasn't further corroded, although I also took measures to divert the drippings from the feeders away from the tower. 

I would treat metallic painted wood the same as sheet metal.

A thread on another list just now mentioned that JB Weld has powdered iron filler, causing it to arc and break down with high voltage.  Better to use Two-Ton Crystal Clear epoxy when gluing something that is supposed to be an insulator.

[W3RSW]

Also I think that the paint layer was so thin that it couldn't dissapate heat the way a thicker piece of shielding metal would. That might explain the sparklers as the RF field heated the individual flakes as they tried to conduct to each other through the insulation of the paint between flakes.

hot time in the old town tonight.

yeah, flakes.....
uh huh. 

[Bill, KD0HG]

A quick, easy test is to spray paint a small test piece of material, let it dry and put it in a microwave oven. On a paper towel (LOL) Give it a few seconds of RF and see if it gets hot. If it sparks or arcs, end the test fast and don't even use the coating with HV DC.
This is also a good test to check insulating materials for RF lossiness.

[w3jn]

No radio project is worth a crap unless it involves arcing, fire, and/or smoked components 

[N3DRB The Derb]

why, yes. 

novice, I am busy in the garage cutting link coil supports and drilling same for #8 solid.

[K1JJ]

No radio project is worth a crap unless it involves arcing, fire, and/or smoked components 

Yep, so true, Johnny.  That feeling you get when it first gets turned on and craps out. Some smoke, a loud 60 cycle hum, few pops, etc.  But then the real satisfaction comes a week later when you confidently walk in the room, and switch on the power just knowing everything will function perfectly.  You just dare it to crap out, but it can't. 


Don:  Yes, I looked at the label and saw no evidence of metal in the paint. I was wise to this before using it, but it got me again.... sigh


Bill: The microwave with a sample sprayed on paper is a great idea, OM!

Rich:  The metal sheet vs: the tiny flakes is a good explanation of what happened.

T

 

[WA1GFZ]

Tom Vu wins the most 4-1000s in one shack award.

[KB5MD]

Some types of paint other than aluminum color are also conductive at
high voltage.  I sprayed a microwave oven transformer with black
paint from an aerosol can and the thing lit up like a fourth-of-July sparkler when I turned on the power.  Black paint and some other dark colors contain carbon black which is very conductive.  Some types of aluminum paint will explode if mixed on a paint shaker.

[WBear2GCR]

The binders have changed.

They are now mostly water borne plastic type materials.
The older paints were mostly less binder and more metal I suspect.
The new plastics are good insulators until you pump some RF around then the metal particles start to do their thang, and the whole bit looks like a bad cap going into breakdown.

My theory.

          _-_-bear

[ka3zlr]

On the Same line, doing a refurb on the P.S. on the 813 machine, replacing the ohmite Bleeder resistors, the phone rings i'm in my swivel chair i bought for the bench.. resistor in hand i spin to answer the phone forearm hits the frame of the P.S. resistor goes sailing through the air, bumble the phone try to catch the resisitor...CRACK...on the floor...

It was one of those moments ya know...