Coating a Weldment Without Ruining the Faces You Just Machined
Coating is the last operation and it is where machined weldments most often get damaged. A frame that measured perfectly can come back with paint in a bore, zinc on a mating face or a part that moved in the galvanising kettle.
Most of that is avoidable, and all of it is decided at quote time rather than at the paint shop.
Masking is a line item
Every machined face, every bore, every tapped hole and every dowel location has to be masked before coating, then unmasked after. It is manual work, it takes time and it is charged.
What that means for the drawing:
- Mark which faces and features are masked. Do not assume the coater knows which of forty flat surfaces are the machined ones. Colour them on a marked up view.
- Say what the acceptance is. A clean masked face with a sharp edge, or is some overspray at the boundary acceptable.
- Say what happens to threads. Plugs, bolts or tape, and whether threads are to be chased after coating.
- Say whether machined faces get a preservative. A bare machined steel face will rust between the shop and your floor. Ask for a rust preventive oil or a peelable coating and say which.
A frame with two large machined faces is cheap to mask. A frame with forty tapped holes on five faces is not, and that difference belongs in the quote rather than in a surprise.
The sequence question
For painted parts the normal order is:
- Weld
- Inspect the welds, because surface NDT methods cannot see through coating
- Stress relieve where required
- Machine
- Blast, prime and paint with machined faces masked
- Protect the machined faces
Blasting before machining rather than after is worth noting. Blasting an already machined face will destroy the finish, so the blast has to happen before machining or the machined areas have to be fully protected, and full protection during blasting is harder than during painting.
Hot dip galvanising needs its own plan
Galvanising is not paint. The part is cleaned in acid, fluxed, then dipped into molten zinc at around 450 degrees C and pulled out.
Three consequences:
The part moves. It goes from ambient to 450 degrees C in a few minutes, unevenly, hanging from chains. Thin sections heat faster than thick ones, residual stress releases, and long slender parts can come out visibly bowed. A machined feature held to a tenth of a millimetre will not survive this.
Everything gets coated. Zinc goes on every exposed surface, including inside bores and on threads. Masking hot dip galvanising is far harder than masking paint, because the masking has to survive acid and 450 degrees C.
Closed sections must be vented. Any sealed cavity will trap air or moisture and can burst in the kettle. Vent and drain holes are a safety requirement and they have to be designed in, which is a conversation with the fabricator before welding rather than after.
So on a machined and galvanised part the sequence is usually reversed: fabricate, galvanise, then machine. The machining removes the zinc where it goes, which is what you wanted anyway, and the part has already done its moving before the tolerances are cut.
That has a cost: you are machining through a zinc layer, and machined areas are then uncoated steel that needs a repair coating or a preservative. It is still normally the right order, and it needs to be agreed before anything is welded.
Paint systems in one paragraph
For most industrial weldments the specification is a surface preparation standard, a primer, and one or two topcoats, with a total dry film thickness. Zinc rich primers give better protection and cost more. The environment the part lives in drives the choice, and there are published corrosivity categories that make this a solved problem rather than an opinion.
What matters for a machined weldment is less the paint system and more the masking and the sequence. Specify the system by reference, then spend your drawing effort on which faces stay bare.
Stainless does not get painted
A stainless weldment gets pickled and passivated instead, and there is a sequence question there too, because pickling attacks machined surfaces as well as welded ones. Usually the answer is pickle the welded structure, machine, then passivate.
What to put in the RFQ
- The coating system, by reference to a standard where possible.
- A marked up view showing masked faces and features.
- Whether threads are masked, chased after coating, or both.
- The preservative for bare machined faces and how the part is packed.
- For galvanising: confirmation of the machining sequence, and that vent and drain holes are permitted where the fabricator needs them.
Five lines, and the part arrives usable.
Send your drawing with the masking marked and we will match it with shops that coat and machine in the right order.
Frequently asked questions
Can a machined weldment be hot dip galvanised?
Yes, and it has to be planned. The part goes into molten zinc at around 450 degrees C, which will move it and which coats every surface including machined ones. On parts with tight machined features the usual answer is galvanise first, then machine.
Who pays if paint ends up on a machined face?
Whoever failed to specify or apply the masking, which is why it belongs on the drawing. State which faces and threads are masked and what the acceptance is, and the argument does not happen.
Is powder coating suitable for large weldments?
It depends on whether the part fits the oven and whether it can take the cure temperature without moving. On large fabricated frames wet paint systems are more common for exactly that reason.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that weld and machine it in house, and we reply within 24 business hours.
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