How to Sand 3D Prints: One Grit, Wet, Then Let the Primer Work

People look at my figures and the first thing they ask is which printer I use, because the prints come out so clean. The printer has less to do with it than they think. They come out clean because I spend a lot of time sanding them. Some printers do a better job than others, but if you want a piece to look pristine, you are going to sand it. That is the only way you get that look.

This is how I do it on resin prints, on the figures I sell and on the painted masters that go to factories. One grit, wet, in a particular order, then a wash, a dry, and a filler primer that does the rest. Sanding is the last stage of sculpting. You had your digital sculpt; this is the part you do by hand.

The short version. Wash the print, dry it, take the supports off, cure it. Then 400 grit, wet, and nothing coarser on a visible surface. Supports first, then the print lines, then every lump, all at 400, because a finer grit only softens a lump into a hill. Wash the part in soapy water with a toothbrush, dry it, then filler primer. The primer shows you what you missed and fills what is left.

Before you sand

Most of what makes sanding easy or miserable was decided before the print finished.

• Supports go where you will sand anyway, or where nobody will look. A support on a plain face costs a minute at 400. A support in a stitched beanie costs the stitching. Small touch-point supports, more of them, leave far less scarring than a few large ones, and a sharp edge wants several so it stays sharp. I wrote the placement decision up here: Where to Place Supports on a Resin 3D Print
• Wash in two baths. A dirty bath of used isopropyl alcohol first, to take the bulk of the resin off. I put the whole build plate in. Then the washer with cleaner alcohol, 5 to 10 minutes for a standard resin. Do not leave parts soaking; forgotten parts crack.
• Dry, then supports off, then cure. I take supports off before the final cure, so if I find a fault I can still fix it with resin. My tool for most parts is a small heat gun, the kind that stands on the table: warm the part until it flexes a little, and the supports come away in your fingers. Thin details stay with the cutters, because a heat gun will take an eyelash off with the support. Tiny parts, hold them in tweezers. Whatever you do, do not force a support off. You chip the piece and give yourself more work.
• Fill the drainage holes with the same resin the part was printed in, in thin layers, each cured under a UV light, slightly overfilled so there is no valley to chase later. Keep at least one hole open until the part is cured, or it balloons and cracks in the booth. Wipe the oily surface with alcohol before curing.

What you need

• Gloves. Resin is bad for your skin, and later you do not want grease on the part.
• Magnifying glasses, headband or spectacles. Some supports are small and print lines at 25 or 50 microns are hard to see on translucent resin.
• Wet-and-dry sandpaper, the grey kind. 400 grit. Cut it into small sheets.
• A tray of water and a towel. You are going to make a mess.
• A cutter, for the support you missed.
• A 120 grit sanding sponge, for connectors only. Never on a surface.
• A hard toothbrush and a window cleaner or dish soap, for after.
• A fruit dehydrator with a temperature control and a timer. Mine dries the parts, and later the primer.
• For the primer coat: a pencil, a torch, fine sanding sponges, and a mask or a dust extractor. A linear sander if you have one.

One grit: 400, wet

https://www.youtube.com/watch?v=RqwFFt_HPjc

I start at 400 and I never go lower on a surface. Coarser paper scratches the resin, and then you spend the rest of the day taking your own scratches out. 400 removes a support mark, a print line and a lump, if you give it the time.

Wet, always. Dip the paper, sand, rinse the paper, sand. Water carries the resin dust off the part instead of grinding it back in, so the surface scratches less, and it shows you the surface honestly. Apply a fair amount of pressure; you are pushing on the piece, not polishing it. On a dome or a sphere, go in circles. For a crevice or the edge of a drainage hole, fold the paper a few times so you have a stiff edge that gets in.

Change the paper with every part. Do not try to save on sandpaper; you lose more time than the sheet costs. Change the water when it goes murky, or you pick the old dust back up on the paper.

The order on each part

1. Supports first. Find them, by eye and by feel, and take the marks down flush. Wipe the part often; wet resin hides defects.
2. Print lines and islands next. Every printer leaves some, from the pull when the plate lifts. A Formlabs leaves fewer than a budget printer; none leaves zero. Knowing how the part was oriented tells you where to look. Sand along the line, then across it. You can hear a print line under the paper; when the sound goes smooth, it is gone.
3. Then the lumps. Hold the part to the light. A support mark you sanded in a hurry leaves a hill around it; a line you softened leaves a valley. Get rid of every one now, at 400. This is the part people skip, and it is why their pieces look bumpy under paint: a finer grit does not remove a lump, it rounds it off and makes it bigger. Going from 400 straight to a fine grit is how you get hills.
4. Leave the inside alone. I do not sand interior surfaces, except a support that stops two parts from fitting. Test-fit as you go, and listen for anything that catches.

Do not hurry this stage. The more time each piece gets here, the less time you spend fixing it later. An extra minute on a lump saves ten under primer.

Connectors

A peg or a socket that will not fit is the one place for coarse paper. A 120 sponge takes half a millimetre off a connector in a few strokes. Keep it off anything you will see. Connectors also have to be the cleanest part of the piece: anything that keys, clicks or holds a magnet gets no support marks and a proper test fit.

Filling what sanding will not fix

Sanding takes material off. Three faults need material put back.

• Pits, pinholes and small chips: a drop of the printing resin from a toothpick, cured under the UV light, then another, slightly overfilled, then sanded flush. A toothpick beats a squeeze bottle, because the bottle pushes air in and a bubble you cannot see will show up the moment you prime.
• A burst or a crack: two-part epoxy putty with a ten-minute working time, pressed into the crack and sculpted flush. Then a skin of UV resin over it, so the two materials sand and prime as one. A fixed piece is a test piece, not a sale.
• A valley you missed: more 400, wet. Filler primer will hide a scratch, not a valley.

Wash it again, then dry it

Sanding dust sits in every crevice, and dust left under primer turns into tiny cement blocks you will be chipping out with a knife. So every part gets washed: water with a window cleaner or a dish soap, a hard toothbrush into the stitching and the folds, then a rinse in clean water. Clean the brush too, or you rinse soap back in. Do not leave parts soaking. Check the water before you throw it out; I have lost more small parts that way than I want to admit.

Then dry. A lint-free cloth works. I use a fruit dehydrator, a couple of hours, because it dries the inside of hollow parts and does not leave fibres. Shake the water out of hollow parts first. If your water is hard, you may find a white powder on the parts afterwards. A brush takes it off.

Then the primer does the rest

This is why there is only one grit. The first primer coat is a filler primer, Mr. Surfacer 500, thinned one to one or one to two with Mr. Color Leveling Thinner. Think of it as a liquid putty. It goes on, dries in about an hour in the dehydrator or two to three hours in the air, and gets sanded off again, which takes the last of the fine scratches and the small marks with it.

The primer is also the inspection. Grey, matt, and every line, lump and missed support shows; resin lets the light through, primer does not. Before I sand it, I go over every part with magnifying glasses and a torch and mark what is left in pencil. Not a marker: the ink stains the primer and comes through the paint, worst under white.

The 500 coat is sanded dry, with a linear sander. I use a Proxxon, and a lighter cordless one on parts with detail I do not want softened. The sanding pads that come in the box are useless and will ruin the print, so I cut a fine sanding sponge, about 500 to 600 grit, into small squares and hot-glue one onto each of the sander's heads. When a pad stops cutting, rip it off and glue on a new one. Marked areas first, then one light pass over the whole part for an even surface; the edge of the sponge gets into crevices. Wipe the dust off with a brush as you go, so you can see what you missed. No sander, the same sponge by hand does the same job, only slower.

Feel the part afterwards: it is smooth, and the faint lines you can still see are the filler sitting in the old scratches, not new ones. That is what the 500 is for. Then the parts get washed again, more thoroughly than the first time, because primer dust in water hardens in the crevices: soapy water, the toothbrush, a rinse, a lint-free towel, and a night in the dehydrator at 45°C. Then a fine primer, 1,200 or 1,500, for the smooth touch. The 500 feels like a surface sanded at 400, the 1,500 like glass. If it looks right in primer, it will look right in paint.

https://www.youtube.com/watch?v=rAlQH05daoE

Wet sanding is module 3 of the post-processing course; module 4 is the filler primer, checking the parts and sanding them with the linear sander, and module 5 the fine primer and which paints go over it: the post-processing course

Six ways to ruin the finish

1. Starting below 400 on a surface. You spend the day removing your own scratches.
2. Jumping grits. 400 straight to fine paper leaves hills.
3. Leaving a lump for the finer grit. It does not remove it; it rounds it off.
4. Saving sandpaper. A tired sheet polishes instead of cutting.
5. Priming over dust. Cement blocks.
6. Closing every drainage hole before curing. The part balloons and cracks.

Safety

https://www.youtube.com/watch?v=uf9idLXAiIc

Gloves, because resin is bad for your skin, and a cured print that feels tacky is not cured. Safety glasses when you cut supports; they fly. Wet sanding keeps the dust out of the air, which is the main reason I do it on the resin. The primer coat is sanded dry, and that dust is resin and primer together, so wear a mask, or put a dust extractor beside the part: the bladeless kind sold for nail salons, with a filter you change each session. On a long session I wear the mask as well. Keep the UV light off your skin.

Questions people ask

What grit should I sand 3D prints with? 400, wet, for resin. I never go lower on a visible surface. A 120 sponge is for connectors only.

Wet or dry? Wet for the resin, always. Dry for the filler primer: a fine sanding sponge, about 500 to 600 grit, on a linear sander or by hand, with a mask or a dust extractor.

Do I remove supports before or after curing? After washing and drying, before the final cure. The resin is slightly soft, the marks are shallower, and if you find a fault you can still fix it with resin before you cure.

How do I get rid of layer lines on a resin print? 400 wet, along the line and then across it, until you cannot hear it under the paper. What is left after that, the filler primer takes.

Can I skip sanding and just use filler primer? No. The 500 is a putty for the last fine marks, not for support scars and print lines. Prime over those and you get a smooth lump.

Why does my print look bumpy after sanding? You went to a finer grit with a lump still there, or you jumped from 400 to a high grit. Back to 400, wet, until the surface is flat in the light.

Do I need a dehydrator? No. A lint-free cloth and patience work. It saves me hours a week and dries the inside of hollow parts, which a cloth cannot.

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It is not an exciting stage, and it is the one that separates a print from a collectible. If you want the whole workflow in one place, from washing and curing through sanding, primer, airbrush and sealing to packaging, that is what the post-processing course is for: the post-processing course

Oasim / Karmieh Designer Toys