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This could be a very stupid question, but when it comes to RF you never know.

Does painting (non-conductive) over the top of a grounded metal enclosure affect its ability to absorb/block RF interference? At the mechanical fixings, the enclosure is not painted and has a good ground connection.

I was wondering because I remembered that RF currents travel on the surface of conductors. Would they just travel "beneath" the paint?

Thanks.

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    \$\begingroup\$ What kind of paint? \$\endgroup\$
    – Eugene Sh.
    Commented Feb 28, 2019 at 14:55
  • \$\begingroup\$ Non-conductive paint \$\endgroup\$ Commented Feb 28, 2019 at 15:03
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    \$\begingroup\$ This is a qualified it depends. If you paint over a ground plane then no. If it's part on an enclosure and the the paint acts as an insulator between the lid and the base, both conductive then yes. This is not a detailed explanation of why but I will leave that for a proper answer. \$\endgroup\$ Commented Feb 28, 2019 at 15:14

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Does painting (non-conductive) over the top of a grounded metal enclosure affect its ability to absorb/block RF interference?

No, the idea of an EMI enclosure is to create a faraday shield around something to block electromagnetic waves around whatever it is enclosing. The shield itself needs to be conductive to be effective and have adequate skin depth, on the inside of the shield the electric field is zero (assuming there are no radiators on the inside). (The skin depth needs to be larger for a faraday shield to be effective, but for most metals anything more than 5mm is effective against anything above 0.1Hz). And watch out for anodization also, which is an oxide layer on aluminum and not very conductive.

For a faraday cage to be effective, it needs to be conductive and continuous as possible (slots and apertures can provide inlets for electric fields to bleed in). This also means that conductivity needs to be maintained at the seams of the enclosure by not painting junctions, using conductive gaskets and flanges that cover seams. (and any shield\conductor interfacing to the enclosure also needs good conductivity and no paint)

As long as the surface of the enclosure is conductive and continuous, it will have the same blocking power even with materials of different conductivity on the outside. If anything the paint will attenuate high frequencies or change the reflection (because any two materials that have a boundary have reflection and transmission coefficients).

In short, adding paint to the surface of a metal enclosure will not change the conductivity of the metal underneath (unless there is some kind of severe chemical reaction, which is unlikely). And at the end of the day it is the conductivity of the metal enclosure that matters most.

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    \$\begingroup\$ It's probably worth explicitly adding that you need to be sure there is NOT paint in the seams between parts of the enclosure. \$\endgroup\$
    – The Photon
    Commented Feb 28, 2019 at 17:57
  • \$\begingroup\$ I didn't because the OP mentioned it, but a good idea nonetheless \$\endgroup\$
    – Voltage Spike
    Commented Feb 28, 2019 at 18:12
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    \$\begingroup\$ Can't have paint between any wire/connector used to ground the cage and the cage, either. \$\endgroup\$ Commented Feb 28, 2019 at 19:31
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    \$\begingroup\$ "…anything more than 5mm is effective…" — did you mean a different thickness here? 5 mm is far thicker than typical shielding components or sheet metal enclosures. \$\endgroup\$
    – Kevin Reid
    Commented Mar 1, 2019 at 5:58
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    \$\begingroup\$ At 0.1Hz if your into mostly blocking 0.1Hz you'll need a 5mm conductor. That is typically the lowest people care about. Noise is specified in datasheets down to 0.1Hz (I care about 0.1Hz a great deal since my signals run into the 0.001Hz range). At 50\60Hz you'll also need about 4-5mm with copper or aluminum. \$\endgroup\$
    – Voltage Spike
    Commented Mar 1, 2019 at 7:25
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Well, although laptop2d answer is correct, if you interpret the word "ability" in a somewhat different sense, some kind of paints could actually improve the overall shielding provided by the underlying metal enclosure.

In fact the paint could reduce the incoming RF wave energy, dissipating it as heat. That's the principle behind what are called RAM (Radiation Absorbent Materials), which are used, for example, on stealth vehicles.

Such paints can also be non-conductive. For example, microscopic iron balls are coated in an insulating film and then deployed on the surface to coat embedded in an insulating epoxy matrix. So, overall, the coating is non-conductive.

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    \$\begingroup\$ They actually make a wide variety of conductive paints that can block RF in various frequencies (especially 2.4ghz) nowadays \$\endgroup\$
    – Voltage Spike
    Commented Feb 28, 2019 at 16:41
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    \$\begingroup\$ yep, Hammond even makes ABS enclosures that are coated on the inside with that. \$\endgroup\$
    – user156429
    Commented Feb 28, 2019 at 17:39

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