Finishing Resin Prints

Fully cured first

A part is not finished being a skin hazard just because it is solid. Formlabs' resin safety FAQ explains that acrylic monomers are allergens in liquid form and inert once fully polymerized, and says gloves should be worn whenever handling liquid resin or unwashed parts, regardless of the material, that an incompletely cured part can still cause a reaction, and that cured parts are not automatically safe to handle.

1. Wash the part as your resin maker says, and let it dry fully. Formlabs says parts must be completely dry before post-curing or post-processing, and gives at least 30 minutes for parts washed in IPA Maker spec.
2. Post-cure it to the resin maker's time and temperature. Formlabs notes that some resins need post-curing to reach their mechanical properties.
3. Check it. A tacky or soft surface means it is not done. See Resin Prints Sticky After Curing.
4. Keep gloves on until you are sure. Washing and curing are covered in Washing and Curing Resin Prints; gloves and eye protection in Resin PPE.

Do not sand a part that is still tacky. Sanding uncured resin spreads it onto your paper, your tools and your skin.

Dust and slurry: the resin's SDS

Your resin's SDS sets the protection. Formlabs' Grey Resin SDS is a typical example: it says to avoid breathing "dust/fume/gas/mist/vapors/spray", to wear protective gloves and eye protection, to provide ventilation that keeps airborne vapors, mists and dusts below workplace limits, and to wear a respirator if ventilation cannot do that. Its disposal section says not to discharge the material into public wastewater or surface waters.

For sanding, Formlabs' support-mark guide adds:

• "Wear a face mask to prevent breathing in dust." Wear gloves, or wash your hands thoroughly afterward.
• Wet sanding with water or mineral oil reduces dust and gives a finer finish; wear gloves to keep dust and sanding liquid off your hands.
• Wear eye protection and gloves during support removal, since some methods break off small pieces.

Wet sanding turns dust into a slurry. As a precaution, collect it, and the water or IPA you rinse parts with between grits and before priming, on paper towels or in a tray, never letting it reach a drain, and dispose of it as resin waste. See Resin Waste and Disposal.

Support marks first

Cut supports close to the surface with flush cutters, as in Removing Supports Cleanly, then sand the nubs. Formlabs' grit sequence for support marks:

Stage	Grit (Formlabs) Starting point	Purpose
Remove nubs	150 to 220	Level the support touchpoints
Refine	400 to 800	Remove the scratches from the first stage
Finish	1000 to 2000	Smooth or glossy surface, depending on how far you go

Start with the roughest grit and work finer. Rinse dust off between grits with water or IPA from a squirt bottle, or brush it off with a toothbrush. Small files and tweezers help on highly detailed parts, and Formlabs warns to be careful with delicate parts so small features do not break off.

Polishing without paint

For a bare resin finish, Formlabs suggests:

• Repeating with progressively finer grits for a smoother, glossier surface.
• Wiping with a microfiber cloth and a few drops of mineral oil, then buffing, for a dull shine.
• A very thin coat of resin cured in ambient light or a curing unit. If the layer is too thick, Formlabs says it will not cure completely, which brings back the uncured-resin problem above.
• Plastic polishes or a UV-resistant clear coat.

Priming and painting resin

Resin takes primer and paint well once it is clean, dry and cured. Formlabs' painting guide for resin parts:

1. Remove supports and sand the support marks, then wash off sanding dust with water and a soft brush (distilled water if your tap water is hard). Dry fully, then use a tack cloth.
2. Spray primer rather than brushing it. Two coats: a thin base coat, then a light final coat that stops when the color is opaque Starting point.
3. Fix flaws the gray primer reveals with fine paper or modeling putty, clean, and prime again.
4. Paint in thin coats and let the paint harden for at least a week before handling Starting point.

Formlabs calls for a NIOSH-approved respirator, a well-ventilated workspace and nitrile gloves while spraying. The primer or paint SDS sets the respirator cartridge type; see Filling and Priming and Painting 3D Prints.

What finishing does to a resin part

• Sanding removes material. Resin prints often carry fine detail; sand nubs, not the surrounding surface, and avoid power tools on small features.
• Primer and paint add a film. Formlabs notes that heavy primer may obscure intricate detail. Keep coats thin on miniatures and on any face that has to fit another part.
• Supports belong on hidden faces. Every touchpoint you keep off a visible face is one you do not have to sand. See Resin Orientation and Supports.