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Primer Coating

Paint, powder & polymer coatings · Spray applied

Primer coating is an intermediate film that promotes adhesion, seals porosity, levels minor defects, or improves corrosion protection before paint.

3D printingStandard cost $$Insulating
Quote a part with PrimerCompare paint, powder & polymer coatings
Primer Coating on a machined part

What is Primer?

On 3D-printed parts it creates a consistent surface for sanding and color coating.

How it works

The cleaned printed part is dried and abraded, then coated with a resin selected to bond both the substrate and the next layer. High-build primers contain solids that fill shallow texture and can be sanded; adhesion primers use chemistry or surface activation to bond difficult polymers. Multiple thin coats reduce solvent trapping in porous prints. Primer is not normally the final weathering surface and must remain within its specified recoat window.

Why specify it

  • Seals porous SLS, MJF, and filled printed surfaces
  • Improves adhesion between polymer and topcoat
  • Provides a uniform base color
  • Sanding primer reduces visible layer texture

Where it falls short

  • Primer does not remove major layer lines or dimensional defects
  • Excess build rounds edges and fills text
  • Trapped solvent can blister under topcoat
  • An exposed primer is usually not a durable final finish

Primer at a glance

Primer is a middle-of-the-range finish; its best score is cosmetic quality (35 of 100), at a standard cost. It adds little lubricity. Surface hardness is not rated: the finish leaves the part's own surface.

Corrosion protectionLow25 / 100
Surface hardnessNot rated—
Cosmetic qualityFair35 / 100
LubricityMinimal10 / 100
Relative costStandard20 / 100
CorrosionHardnessCosmeticLubricityEconomy

Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.

Primer key numbers

Typical values from the governing specifications and shop practice; bars are this finish against the highest value among paint, powder & polymer coatings finishes.

Thickness15–50 µm0.59–2 mil
Thickness
15–50 µm after sanding for a sprayed cosmetic primer; high-build systems may require a separate specification
Electrical
Insulating
Colours
  • gray
  • white
  • black
  • beige
Appearance
Uniform matte base intended for sanding or overcoating, not final cosmetic service.

Specifications for Primer

The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard.

Drawing call-out

SANDABLE EPOXY PRIMER, GREY, 20–40 µm DFT AFTER SANDING; TOPCOAT WITHIN MANUFACTURER RECOAT WINDOW; MASK THREADS, TEXT, AND DATUM FEATURES

Which materials take Primer?

Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.

Material familyVerdictNotes
Aluminum
YesCommon metal primer substrate after degrease and conversion or etch pretreatment.
Composite
YesPrimer can seal exposed fibers and filler after preparation.
DMLS / SLM
YesMetal AM parts are primed after powder removal and any sealing.
MJF
YesClean blasting residue before priming.
Polymer
YesChoose adhesion chemistry for the exact resin.
SLA
YesFully post-cure and remove uncured resin before primer.
SLS
YesPorous nylon usually needs controlled sealing and thin coats.
Steel
YesStandard corrosion-inhibiting primer substrate after blast or phosphate.
Brass
With careNeeds a stain-blocking primer.
Copper Alloy
With careTarnish isolation and adhesion must be validated.
FDM
With careLayer lines require mechanical preparation; solvent compatibility varies.
Magnesium Alloy
With carePrime promptly over a qualified conversion coating.
Stainless Steel
With careNeeds abrasion and an adhesion-promoting primer.
Titanium
With careMechanical profile and a compatible primer are required.
Zinc Alloy
With careUse a zinc-compatible primer; outgassing can blister.

How Primer is applied, and what it does to the part

  1. 1Clean and dry
  2. 2Abrade
  3. 3Activate if required
  4. 4Mask
  5. 5Spray thin coats
  6. 6Flash
  7. 7Cure
  8. 8Sand
  9. 9Clean
  10. 10Apply topcoat within recoat requirements
DimensionsNet 15–50 µm film can soften small lettering and reduce clearance; high-build filling operations require separate dimensional allowance.
Build-up per side15–50 µm — Net buildup is the cured thickness remaining after sanding.
MaskingUsually needed. Mask threads, fits, fine text, vents, contacts, optical areas, and adhesive interfaces.
ElectricalInsulating
AppearanceUniform matte base intended for sanding or overcoating, not final cosmetic service.
Added lead time2–5 working days on top of the bare part

Design rules for Primer

What to decide on the drawing before the part goes to the finisher.

  1. Name the topcoat together with the primer.Intercoat adhesion depends on chemistry and recoat window.
  2. Define thickness after sanding.Applied primer and retained primer are not the same buildup.
  3. Keep high-build primer out of small holes and text.Filler rapidly obscures fine geometry.
  4. Dry porous prints completely before priming.Moisture and solvent trapped in pores cause bubbles and delayed failure.

Primer versus the alternatives

Finishes engineers weigh against Primer, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
Primer
this page
25—35$$15–50 µm—
Clear Coating
Primer vs Clear Coating →
201285$$15–50 µmwhen the substrate is already cosmetic and only transparent protection is needed
Wet Painting
Primer vs Wet Painting →
591090$$20–80 µmwhen a complete primer-and-color system should be specified together
CorrosionHardnessCosmeticLubricityEconomy
PrimerClear Coating

The radar overlays Primer with Clear Coating, its most common alternative — chosen when the substrate is already cosmetic and only transparent protection is needed. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Primer Coating is a standard-cost finish ($$): cheaper than about 78 % of the finishes FabDigit quotes.

Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates. The exact figure appears on the instant quote once the part is uploaded.

Lead time and compliance

Added lead time2–5 working days
RoHSCompliant
NotesConfirm pigment, solvent, and curing-agent compliance for the selected primer.

What Primer is used for

  • Painted prototypes
  • SLS nylon models
  • MJF housings
  • SLA display models
  • Composite-filled prints

Primer — frequently asked

do 3D prints need primer before painting

Often yes, especially porous SLS or MJF parts and sanded SLA models. Primer seals the surface, exposes defects, and gives the topcoat a consistent base.

how thick is primer coating

A cosmetic sprayed primer may leave about 15–50 µm after sanding. High-build filler primers can be thicker and need their own dimensional specification.

does primer hide layer lines

It fills only shallow texture. Pronounced layer lines require sanding or another smoothing process before the final 15–50 µm primer layer.

can primer be used as a final coat

Usually no. Primer is optimized for adhesion and filling, not ultraviolet stability, stain resistance, or final appearance.

why does paint peel off primer

Likely causes include contamination, incompatible chemistries, missed recoat window, inadequate cure, or sanding dust. Qualify the complete primer and topcoat system on the actual substrate.