Filling and Priming

Fillers add thickness

Sanding removes material; fillers and primers add it. Both change the part. A filler primer is designed to build a layer thick enough to bury fine scratches and layer lines, which is also thick enough to soften fine detail and change a tight fit:

• Per-coat thickness is measurable. Rust-Oleum's data sheet for its aerosol Filler Primer gives a recommended dry film thickness of 1.5 to 2.5 mils (37.5 to 62.5 micrometers) per coat Maker spec. That is that product's figure; yours will be on its own data sheet. Several coats add up.
• Detail gets buried. Formlabs notes that heavy primer can replace sanding but "might obscure any intricate details."
• Fits get tighter. Keep filler and primer out of holes, slots and mating faces, or mask them. Re-check the fit after finishing. See Fits and Tolerances.

Choose the filler by the job

Polymaker's sanding guide compares three common gap fillers:

Filler	Use it for	Trade-offs (Polymaker)
Automotive body filler	Large seams and gaps, functional or load-bearing parts	Hard to sand by hand; needs power tools, which risk melting PLA. Apply sparingly and avoid sharp inside corners.
Lightweight spackle	Display models, fine gaps	Spreads by hand and sands easily with fine paper, but dents and scratches easily. Not for mechanical parts.
Model putty	Small to medium seams, cosmetic repairs	Between the other two in strength and sanding effort

Polymaker adds a rule worth keeping: test fillers and adhesives on a scrap print first, to check compatibility and finish.

Safety: each product's SDS

Fillers, primers and their solvents are chemical products. Read the SDS for the exact product you buy (search the maker's site for the product number on the can) and follow it. As examples of what those sheets contain:

• Aerosol filler primer. Rust-Oleum's SDS for its gray aerosol Filler Primer (249279) classifies it as an extremely flammable aerosol in a pressurized container that may burst if heated, causing serious eye irritation, possibly drowsiness or dizziness, suspected of causing cancer and may damage organs through prolonged or repeated exposure. It says to keep it away from heat, sparks and open flames, not to spray on a flame, not to breathe the spray, and to use it only outdoors or in a well-ventilated area, with gloves, protective clothing and eye protection. It names an air-purifying respirator with organic vapor cartridges where exposure limits may be exceeded.
• Sanding a filler. The SDS for Gorilla Wall Repair (a spackle-type patching compound) says airborne particles may irritate the eyes and respiratory tract, and that dry sanding calls for an approved dust respirator and eye protection.
• Body fillers and model putties have their own SDS; read it before you open the product.

For general ventilation in a print room, see FDM Emissions and Ventilation and Setting Up a Safe Workspace.

The fill, prime, sand loop

1. Sand off the worst. Remove support scars and high spots first. See Sanding 3D Prints.
2. Clean the part. Formlabs washes parts with water and a soft brush and lets them dry before using a tack cloth.
3. Fill gaps and seams with a thin layer of filler. Let it dry, sand it, and repeat until the gap disappears. Polymaker advises applying body filler sparingly to keep later sanding down.
4. Spray a filler primer. Formlabs notes that the neutral gray of a primer highlights imperfections so you can sand or fill them.
5. Mark and fix flaws, sand with fine paper, clean, and spray again.
6. Stop when the surface is even. Every extra coat adds thickness and blurs detail.

Spraying a primer

Follow the instructions on your can; distances, coats and dry times differ between products. Two maker examples show the range:

Step	Rust-Oleum aerosol Filler Primer data sheet Maker spec	Formlabs priming guide Starting point
Where	Outdoors or a well-ventilated area such as an open garage; not near open flame	Well-ventilated space, with a NIOSH-approved respirator (the Rust-Oleum SDS names organic-vapor cartridges)
Conditions	50 to 90 °F (10 to 32 °C), humidity below 85 percent	Not stated
Mixing the can	Vigorously for one minute after the mixing ball rattles	Swirl, do not shake, for 2 to 3 minutes; shaking mixes propellant into the solvent and causes bubbles
Distance	12 to 16 inches, steady back-and-forth strokes	6 to 8 inches, short rapid strokes
Coats	2 or more light coats a few minutes apart	Two in total; stop as soon as the color is opaque
Sanding	Wet sand once dry to the touch; allow 2 to 4 hours before dry sanding	Fine paper (finer than 600) on flaws after the first coat

Formlabs suggests using a primer and paint from the same brand, made for plastics. Rust-Oleum's data sheet lists the filler primer for metal, wood and fiberglass, which is a reminder that hardware-store products are not labeled for printed plastics: test on a scrap print first, as Polymaker advises.

On resin parts

Prime only fully washed, dried and cured resin parts, and remove support marks first. The resin-specific steps are in Finishing Resin Prints.

Painting over the primer is covered in Painting 3D Prints. For a comparison of filling with other ways to smooth a surface, see Smoothing Methods and What They Change.