How to create a white underbase for dark garments

What happens if you skip it

Plastisol is more translucent than it looks, and a garment is a coloured, absorbent, textured surface. Print red straight onto black and you get a dark muddy brown that photographs badly and looks nothing like the proof; print yellow and you get an olive smear. Even heavy opaque inks laid directly on dark cotton go dull and shift hue, and the fix is not more ink — piling on coats gives you a stiff, cracking print that still reads wrong. The underbase is the fix: a layer of white printed first and flash-cured, so every colour after it is landing on white instead of on the shirt. That is why a photographic print on a black tee can look bright rather than sunken, and it is the single biggest difference between a shop's dark-garment work looking professional or looking homemade.

Why the underbase is choked, and by how much

An underbase that exactly matches the artwork outline is a trap. The instant a screen shifts by half a millimetre — and it will — white peeks out from behind the colour and rims the whole design in a bright halo. The fix is to choke it: shrink the white inward by a small, deliberate amount so the colour plates over-print past its edge and hide it. It is worth doing the arithmetic once, because the units get muddled constantly. A point is 1/72 inch, so at 300 dpi one pixel is 0.24 pt — a quarter of a point, not a half. A half-point choke is about 2 pixels at 300 dpi, and 4 pixels at 600 dpi. The working range shops talk about is roughly a quarter to three-quarters of a point, which is 1 to 3 pixels at 300 dpi or 2 to 6 at 600. And the widely taught channel-level method is not equivalent to the light end of that: selecting the solid areas of the underbase channel and stroking them white at 4 to 6 pixels with Location set to Center pulls the edge in by about half the stroke width, so 2 to 3 pixels at 300 dpi — a half to three-quarters of a point, two to three times a single-pixel contract. It also firms up a soft edge, which is part of why it is taught. Pick one method and one number: too little choke and you get the halo, too much and colours go dull at the edges because there is nothing bright left behind them. The right number is a property of your press's registration accuracy, not of the artwork — find it once on a test print and reuse it.

Solid base or tonal base

A flat spot design wants a solid base: threshold the coverage, choke it, done. A design with gradients, halftones or soft shadows wants a tonal base, where the white underneath tracks the density of the colour above it — so a 40% shadow gets 40% white rather than a solid slab that lights the shadow up from below and kills it. Getting this wrong is a common cause of prints that look chalky and flat: a full-strength base under everything means every colour prints at the same brightness and the image loses its depth. Here the underbase is built from the combined coverage of every colour plate and inherits their tone, then erodes inward by your choke value at the output resolution. Because it is generated from the separation rather than drawn by hand, it follows along when you merge inks, change the ink count or delete a plate — you are never maintaining it as a separate piece of art that silently goes out of date.

Highlight white is not the underbase

This is the distinction people conflate, and it costs them prints. The underbase goes down first, under everything, and its job is to turn the shirt white so the colours have somewhere to sit — you never see it directly, and if you can see it, something has gone wrong. The highlight white goes down last, over everything, and its job is the brightest hit in the image: the glint in an eye, the specular edge on chrome lettering, the top of a water splash. They are different screens with different artwork. Trying to serve both from one white plate is exactly how prints come out flat — either your highlights are only as bright as an underbase seen through colour, or your underbase is hot enough to wash out every midtone above it. Here they are two plates, and the highlight white has its own threshold so you decide how far down the tonal range it reaches. Turn it off for artwork with no true whites in it; turn it on for anything with metal, glass, water or a face.

Printing it: flash, mesh and squeegee

The separation is half the job. On press, the base goes down first and gets flash-cured before anything prints on top of it — wet white under wet colour mixes and you lose both. How long you flash and how far you cool afterwards changes the result: an under-flashed base picks up colour, and a base that is still hot when the next colour hits gives you a gummy print that will not release cleanly. Base white is normally printed through a coarser mesh than the colours, because it needs a heavier deposit to actually cover the garment; the colours after it can run finer because they only have to cover white. And a base printed too heavily brings its own problems — a thick, plasticky hand that customers notice and a print that cracks at the fold. The target is the thinnest white that fully covers, which on a rough or heathered garment is more than you would like and on smooth ringspun cotton is less than you expect. That number lives in your shop, not in a separation file.

Choke, spread and trapping, all in one pass

Choke and spread are the same operation in opposite directions: pull a plate in, or push it out. Both are per-ink here, so you can choke the underbase, spread a thin lettering plate that would otherwise starve, and leave everything else alone. Trapping handles the other registration failure — a hairline gap between two adjacent colors — by overlapping them slightly so a small misregistration shows overlap rather than garment. None of it is mysterious; all of it is measured in pixels at your output resolution, so the numbers mean something you can reason about.

Set the garment color and stop guessing

The separation takes the garment color as an input, and the proof renders your plates back over that color — underbase, colors, highlight white, in print order. That is the difference between checking a separation and hoping about one. Look at the proof on your actual shirt color before you burn anything, and if the underbase edge is visible in it, it will be visible on the press.

What it costs

Underbase generation is part of the print-ready render: 10 credits per image, and for 24 hours after that render you can re-render the same image as often as you like, free, for 24 hours. That window matters here more than anywhere, because finding your shop's correct choke value is iterative — you render, look, adjust, and render again — and charging per render would only teach you to avoid the control that most needs using. Be realistic about the clock, though: a day covers a morning of on-screen adjustments and a test print in the afternoon, which is usually enough to land the number. It does not cover a value you chase across a week between other jobs. If you are dialling in a press from scratch, do it in one sitting, or accept that a later session is another 10 credits. Credits come with a monthly plan, or as a one-off pack whose credits never expire — see the pricing page for current prices in your currency. The separation itself, if all you want is flat spot plates, stays free and client-side — as do both PSD exports, including the multichannel one with real spot channels.