Resin prints: what to do, in what order

The printer here is an Anycubic Photon M3 Premium, it’s my dad’s, and most of what’s on this page is something I got wrong first. The order matters more than any single step in it, and almost nothing states the order plainly, so that’s what this is.

Wash. Supports off. Cure. File and sand.

A quick note on the words, since they all turn up below. The build plate is the metal plate the model hangs from. The raft is the flat slab the printer builds underneath the model to hold it on. Supports are the forest of thin spikes between the raft and the model. Curing is hardening the print off under UV light once it’s washed, because a print that comes out of the machine isn’t fully set yet.

First, set the bench up

A print that’s just finished is covered in liquid resin, and liquid resin is a labelled hazardous substance rather than a mild annoyance. It irritates skin and eyes, it causes allergic skin reactions, it irritates your airways, it belongs outdoors or somewhere well ventilated, and it gets stored locked up. That’s the formal classification on the bottle, not a cautious reading of it.

The allergy one is worth understanding, because it works differently from the rest. Ordinary irritation happens while the stuff is on you and then stops. This kind is your immune system learning the chemical, and once it has, it reacts worse every time instead of settling down. Nobody leaves the hobby because resin stung them once.

So, before you lift anything:

• Nitrile gloves, changed when they get resin on them. No glove material is rated to hold resin out indefinitely, and the manufacturers won’t name one, so the answer is a cheap glove you replace rather than a good glove you trust.
• Tubs within reach of the printer. Everything that goes wrong goes wrong while you’re carrying a dripping build plate across a room.
• Kitchen roll down, and a bag for it. Treat the paper as contaminated, because it is.
• Ventilate it, and not in a room you sleep in.

1. Get the print off the plate, and do it soon

Lift the plate out, hold it at an angle over the tank and let it drip for a minute. That resin runs back in and gets reused, so it’s the cheapest minute of the whole process.

Don’t leave the print on the plate overnight. Leftover resin keeps hardening where the raft meets the plate, and after a few hours it has welded itself on. Within an hour or two, a scraper pushed in at a shallow angle from one corner pops it straight off.

If it has welded on, the fix sounds wrong but works: cold, not heat. Put the whole build plate in the freezer for ten to fifteen minutes. The metal and the hardened resin shrink at different rates, so the bond gives way on its own, and then a scraper gets under it. A little IPA run along the join soaks underneath and helps. Warming it does nothing useful.

Never a hammer and chisel. A dent in the build plate is permanent, and your levelling is wrong from then on, which costs far more than the print you were trying to save.

2. Wash it

IPA is isopropyl alcohol, and it’s what dissolves the film of uncured resin off the print. Two tubs if you can, used in the order dirty then clean. The first takes most of the resin off and the second takes off what the first left behind. When the first gets too cloudy to see through, it becomes the second tub’s job and you make up a fresh clean one.

A few minutes in each, swirled around, is plenty for a small part. Get into recesses with a soft brush, and watch anything hollow, because resin pools inside and comes back out later.

Keep a lid on the IPA. Its flash point is about 12°C, so at normal room temperature it’s quietly giving off vapour that will catch light. Nowhere near the kitchen, nowhere near anything hot.

Water-washable resin swaps the alcohol for water and doesn’t get you out of anything, since the water ends up with resin in it.

3. Take the supports off now, before curing

This is the step everyone does in the wrong place, me included.

A washed but uncured print is still faintly bendy. The support contacts are the thinnest part of the whole structure, so they shear off cleanly and leave small flat nubs. Cure it first and the part turns brittle, and the same supports lever little craters out of the surface on the way off. On a detailed model, that’s the detail gone.

• Cut the forest into sections first with flush cutters, which are snips with one flat jaw. A support forest is all joined together, so force at one end travels everywhere at once.
• Cut close to the model, not against it. Leave the last stub standing and take it down flat later. Cutting flush against a surface is how you get a divot.
• Wear eye protection. Support spikes are brittle and under tension, so they ping across the room when they let go.
• Keep the gloves on. The print still has a thin film of uncured resin on it even after washing.

4. Cure it

Now, and not before. A cure machine, a UV lamp, or sunlight at a pinch.

Short, and test it. Minutes, not tens of minutes, for a small part. Over-curing is real and it makes parts yellow and brittle, which is also why a finished print on a sunny windowsill keeps getting worse over months. Cure a test print in stages, look at it between each one, and write your number down for that resin and that lamp. Numbers off the internet are for somebody else’s lamp.

A print that’s still tacky afterwards was either under-cured or not washed properly, and the second is more likely. Another wash usually fixes a surface that a longer cure won’t.

5. File and sand

Now it’s ordinary hard plastic and you can treat it like any other material. Take the support nubs down level with a flat needle file first, because a file only cuts what’s standing proud and stops when the surface is level, then work up through the sanding grades. Keep it wet, for the dust rather than the heat, because cured resin sands to a very fine powder you don’t want in the air. The full method is in how to sand a 3D print smooth.

Cured resin behaves differently under tools from filament, and it catches people out first time. A deburring tool on PLA peels one long curl off an edge. On resin it takes dust and little chips instead, because the material is brittle. Same tool, same stroke, and it isn’t a sign you’re doing it wrong.

Primer sticks very happily to a properly washed and cured resin print, so this is the easiest of the common materials to paint.

Why your model needs drain holes

If the model is hollow, it needs holes at the lowest point so resin can run out. Without them, uncured resin stays sealed inside, and weeks later it finds a seam and leaks out over the part you’d finished and painted. Trapped resin also makes a sealed cup that the plate has to pull against on every layer, which is its own cause of failed prints.

Where does the waste go?

Resin and its container are hazardous waste, formally, and that’s what they have to be thrown out as. So the rule here is simple: nothing leaves the bench while it’s still liquid.

• Failed prints and support forests get cured before they go in the bin. Until they’re cured they’re still wet resin.
• Dirty wash liquid goes in a clear sealed jar. Let the sludge settle, pour the clearer alcohol back into the first-wash tub, and leave the sludge tub in the sun or under a lamp until it’s a solid lump. The lump is ordinary plastic and goes in the bin.
• The leftover liquid and the empty bottles go to a council waste site that takes chemicals, not down the sink and not in the wheelie bin.
• Gloves and kitchen roll get cured too where you can, and binned either way. Don’t leave a resin-soaked towel in an open bin indoors.

None of that is difficult. It’s a routine you either have or you don’t, and it’s the real reason to think twice before buying a resin printer instead of a filament one. There’s a full comparison in resin or filament.

The order, one more time

Step	Why it’s there
Drip, then off the plate	Resin welds the raft on within hours. Cold shifts it, heat doesn’t
Wash, dirty tub then clean	Takes off the film of uncured resin you’d otherwise handle
Supports off	Uncured prints still flex, so contacts shear instead of chipping
Cure	Short and tested. Too long turns it yellow and brittle
File, then sand wet	The file levels nubs, the paper smooths. Wet for the dust
Cure the waste, then bin it	Nothing leaves the bench liquid

Full disclosure: I sell a kit with the cutters, files and sanding sponges for steps three and five. It isn’t a resin kit, it has no gloves and nothing to wash or cure with, and those are what you need first.