Designing for powder coat: drain holes, hanging points and recesses

Coating happens on a moving line

A powder-coated part isn’t sprayed on a bench. It’s hung on a hook, dragged through tanks of liquid pretreatment, sprayed with charged powder, and baked — all while dangling from one point. Design with that journey in mind and the finish is flawless. Ignore it and you get trapped water, bare patches and a hanging mark right where it shows.

Give trapped liquid a way out

The pretreatment stages are wet. Any enclosed pocket — a boxed frame, a folded channel, a capped tube — fills with fluid and, without an exit, carries it into the oven, where it boils out through the fresh coat and ruins it. A small drain hole at the lowest point of each trapped volume lets it empty. It also stops rust brewing inside a sealed box later.

Design in a hanging point

Every part hangs from a hook, and wherever the hook touches stays bare — the contact point can’t be coated. Give us a dedicated hole or tab to hang from, somewhere hidden or unimportant, and the visible faces come out perfect. Leave it to chance and the bare spot lands somewhere you’ll see.

Deep recesses coat thin

Powder is charged and clings to the earthed metal — but that same charge avoids deep internal corners and narrow recesses (the "Faraday cage" effect), so those areas coat thinly or not at all. Sharp inside corners, deep slots and boxed-in channels are the usual culprits. Open them up, soften the corners, or accept that the recess gets less protection than the face.

Say what stays bare

Coating adds real thickness, so anything that has to stay to size gets masked off: threads, mating faces, earth points, bearing bores, sliding surfaces. Mark them and we mask them before the booth. Flag it after, and it’s a rework.

1. 01 Threaded holes A coated thread won’t take its fastener.
2. 02 Mating & sealing faces Anything that has to sit flat to another part.
3. 03 Earth / bonding points Powder is an insulator — earths must stay bare.
4. 04 Bearing bores & slides Sizes that a coat thickness would throw out.