Underbase White in Screen Printing: The Complete Technical Guide

The white underbase is the most important single color in dark garment screen printing. It determines everything that happens on top of it: color accuracy, opacity, vibrancy, hand feel, and print durability. A perfectly executed top color separation printed on a poorly executed underbase will produce a disappointing print. The reverse — a well-prepared underbase carrying mediocre top colors — will produce a result far better than the sum of its parts.

Yet the underbase is treated as an afterthought in many print shops. It goes through the press first, often on the coarsest mesh, with the least attention — and then printers wonder why their colors look dull on dark shirts. This guide treats the underbase with the technical seriousness it deserves: how it works, how to design it correctly, how to print it, and a thorough troubleshooting section for everything that can go wrong.

Why Dark Garments Need an Underbase

Screen printing ink is not infinitely opaque. Even bright white plastisol, applied through a standard 86 TPI screen, will show the dark garment color bleeding through at normal ink deposit thicknesses. Red ink on a black shirt without an underbase looks dark maroon. Yellow on navy looks green. The garment color contaminates every color printed on top of it.

The white underbase solves this by:

1. Creating a neutral, high-reflectance foundation — white reflects all wavelengths of light, giving top colors a bright, accurate starting point rather than absorbing some wavelengths through a dark substrate
2. Blocking substrate color — physically covering the dark garment with an opaque white layer so top colors see only white beneath them
3. Improving color vibrancy — colors printed on white appear more saturated and accurate than the same colors printed on a dark or colored substrate
4. Enabling photographic and process color work on dark garments — without an underbase, CMYK and simulated process separations cannot produce accurate color on dark shirts

Types of Underbase

Not every job requires the same underbase approach. Understanding the different types allows you to choose the correct technique for each design.

Full Underbase

A full underbase covers the entire design area with solid white. Every top color prints on top of white, regardless of whether that top color area would be visible against the dark garment without underbase.

When to use: Photographic or multi-color designs where color accuracy is critical; bright or pastel top colors that have poor opacity; maximum vibrancy required.

Limitation: Creates the heaviest ink build-up and the stiffest hand feel. Multiple thick layers of plastisol can make a shirt feel like cardboard.

Selective / Partial Underbase

A selective underbase covers only the areas of the design where it is actually needed — typically light colors, gradients, and fine detail. Dark colors (black, dark navy, dark brown) may not require underbase because they have sufficient opacity on their own.

When to use: Designs with a mix of dark and light colors; when hand feel is a priority; when press time or number of screens is limited.

How to create: In Photoshop, create the underbase by merging the top color channels, then manually removing areas where the top colors are dark enough to print without underbase.

Choke Underbase (Trap)

A choke underbase is deliberately sized slightly smaller than the top color artwork — typically by 0.3–0.75mm (choking inward). This prevents the white underbase from showing around the edges of top colors if there is any misregistration during printing.

When to use: Always, as a standard practice in professional multi-color dark garment printing. An underbase that is the exact same size as the top color will show a white halo around every top color element if registration is off by even 0.5mm.

How to create in Photoshop: After building the underbase, apply a selection contraction (Select → Modify → Contract by 3–5 pixels depending on print size and registration tolerance) before creating the underbase layer. Alternatively, use minimum filter (Filter → Other → Minimum, radius 2–4 pixels).

Highlight White

A highlight white is a separate white screen used to add brilliant white highlights to a design on top of the fully printed top colors — it is the last color down, not the first. It is not an underbase but is related to white ink management in dark garment printing.

Highlight white creates areas of pure, opaque white detail (white text, white accents, white reflections in illustration work) that would lose their crispness if printed as the underbase.

Underbase Design: Building the Correct Layer

The underbase is a constructed layer — it must be built in Photoshop (or equivalent software) from the top color separations. It is not simply “the full design printed in white.”

Manual Underbase Construction

Step 1: Merge all top color channels into a single composite layer. This creates a layer that represents the total printed area of the design.

Step 2: Remove areas where underbase is not needed. This includes:

• Any area that will be printed with a dark, opaque color (black, dark navy, dark green) that does not need a white foundation
• Any area intentionally designed to use the dark garment as a shadow tone (negative space, intentional blending with the shirt color)

Step 3: Apply a choke (contract) to the underbase selection. Contract by 3–8 pixels (depending on artwork size and registration tolerance of the press).

Step 4: Check for minimum detail size. Fine lines, small text, and tiny design elements in the underbase may need to be slightly thickened (spread/expand) to ensure they print cleanly. An underbase line that is 1mm wide on the film may not survive the flash cure and wet-on-wet printing cycle without spreading.

Step 5: Output the underbase as a separate film. The underbase channel should be a clean, solid black-and-white film — no halftone dots in solid areas.

Underbase for Simulated Process and Photographic Work

Photographic dark garment prints often use a halftone underbase rather than a solid underbase. A halftone underbase modulates the white ink coverage across the design — applying more white in highlight areas (where top colors need a bright foundation) and less white in shadow areas (where the dark garment contributes shadow depth).

This approach:

• Reduces overall ink build-up (less ink in shadows)
• Allows the dark garment to contribute shadow tones naturally
• Produces a softer hand feel than a solid underbase
• Requires more careful registration and more precise underbase design

The halftone underbase is designed by creating a luminosity map of the top color artwork — the brightest areas of the design get the heaviest underbase; the darkest areas get no underbase or minimal underbase.

Ink Selection for Underbase Printing

The underbase must be the most opaque ink on press. Not all white plastisols are equally opaque — and opacity matters more for underbase than for any other color.

Key Properties for Underbase Ink

Opacity: The primary criterion. High-opacity underbase white should cover a black substrate in a single pass through 55–77 TPI mesh with a single squeegee stroke. Test by printing through a single pass and checking coverage while the ink is still wet.

Viscosity: Underbase ink is typically heavier-bodied than color inks — it needs to sit on top of the dark fabric surface rather than penetrating into fibers. Too thin = coverage poor. Too thick = screen clogs, poor lay-down.

Curing characteristics: Underbase must cure correctly under flash cure conditions (short, high-intensity heat exposure) to allow wet-on-wet top color printing. It should be dry-to-touch after flash without fully curing through — a fully cured underbase has reduced adhesion for top colors.

Bleed resistance (for polyester): Standard white plastisol applied to polyester garments causes dye migration — the polyester dye bleeds into the hot white ink during curing, turning white pink, gray, or yellow. On polyester, use specifically formulated low-bleed or bleed-blocking white underbase inks.

Mesh Selection for Underbase

Printing the Underbase: Press Technique

Flash Curing the Underbase

After printing the underbase, it must be flash-cured before top colors print on top. Flash curing:

• Gels the ink surface so top colors don’t mix with the wet white
• Stabilizes the underbase position on the garment
• Allows wet-on-wet top color printing without disturbing the underbase

Flash cure settings: Typically 3–8 seconds at 150–170°C (300–340°F) with a quartz or infrared flash cure unit. The goal is a dry-to-touch surface, not a fully cured ink. Check by touching the edge of the underbase area with your finger — it should not transfer ink.

Overflashing: If the flash cure is too long or too hot, the underbase surface becomes fully cured and smooth. Top colors sitting on an overflashed underbase have reduced adhesion and may crack or peel. The surface should be slightly tacky, not glassy.

Underflashing: If the flash cure is too short, the underbase remains wet. Top colors mix with the wet white, producing color contamination — bright colors go muted, the print looks washed out.

Off-Contact Distance for Underbase

Underbase printing typically uses a slightly larger off-contact distance than top color printing — 4–6mm — because the coarse mesh and heavy ink deposit require a strong snap-off to leave clean edges.

Double Stroke Underbase

For maximum opacity on very dark garments or very absorbent fabrics, some printers use a double stroke on the underbase — two squeegee passes on a single print cycle. This deposits approximately 50–70% more ink than a single stroke and significantly improves opacity.

However, double stroke increases ink build-up and hand feel stiffness. Only use when single stroke is genuinely insufficient.

Underbase on Polyester and Synthetic Garments

Polyester presents a specific challenge: dye migration (also called “dye sublimation bleed”). Polyester dyes are small molecules that penetrate into plastisol ink under heat, turning white ink pink, blue, gray, or whatever the garment color is.

Why Dye Migration Happens

Polyester dyes are thermoplastic — they are driven into the plastisol by heat during the curing process. The hotter and longer the cure, the worse the migration. Flash curing is particularly problematic because flash cure units deliver intense heat rapidly.

Preventing Dye Migration

Use a bleed-blocking underbase ink. Major ink manufacturers offer specific underbase formulations designed to physically block dye migration. These inks contain larger polymer structures that resist dye penetration. They are not optional on bright polyester garments — they are essential.

Reduce flash cure temperature. If possible, use a lower temperature, longer duration flash cure rather than a high-temperature short flash. Dye migration accelerates with temperature.

Cure at lower temperature for longer time. In the conveyor dryer, reduce belt speed slightly and reduce temperature — this achieves full cure with less intense heat spike.

Use a sports grey or similar muted color for athletic prints. The migration problem is worst on bright, saturated polyester colors (royal blue, red, black). Sports grey and other low-saturation colors migrate less aggressively.

Test every garment brand and color. Different polyester dye formulations migrate at different rates. A white underbase that works perfectly on one brand’s royal blue jersey may bleed badly on another brand’s same-color garment.

Troubleshooting: Underbase Problems

Problem 1: Colors Look Dull / Dark on Dark Garment Despite Underbase

Symptoms: Top colors should be vibrant but appear muted or washed out; underlying garment color is visible through top colors.

Causes and fixes:

• Underbase opacity insufficient: Switch to a higher-opacity underbase white; increase mesh count (coarser mesh = more ink deposit)
• Underbase underflashed: Wet underbase mixed with top colors, contaminating them. Increase flash cure time.
• Top colors printing wet-on-wet on underflashed base: Flash more aggressively; do not print top colors on wet underbase.
• Underbase coverage incomplete: Check if underbase film covers all areas that should have white beneath them. Design may be missing underbase under some color areas.

Problem 2: White Halo Visible Around Design Elements

Symptoms: Thin white border visible around design elements where underbase extends beyond the top color.

Causes and fixes:

• Underbase not choked: Apply a choke of 3–8 pixels to the underbase layer to ensure it is slightly smaller than the top colors.
• Misregistration: Press registration is off; top colors are printing offset from the underbase. Correct press registration.
• Underbase and top colors on different size films: Check that all films are output at identical scale.

Problem 3: White Bleeds Through Top Colors (Underbase Showing)

Symptoms: Bright white visible through semi-transparent top colors; colors appear washed out and light.

Causes and fixes:

• Top color ink not opaque enough: Use higher-opacity top color formulations, or apply a double stroke on the top colors.
• Top color mesh too fine: Fine mesh deposits less ink; use a coarser mesh for top colors requiring heavy coverage.
• Underbase overflashed: A glassy, over-cured underbase surface reduces top color adhesion and allows white to reflect back through. Reduce flash cure intensity.

Problem 4: Dye Migration / White Turning Pink or Gray (Polyester)

Symptoms: White underbase (and white top colors) are discolored — pink, gray, or garment color — after curing.

Causes and fixes:

• Standard white ink used on polyester: Switch to a bleed-blocking/low-migration underbase formulation specifically designed for polyester.
• Cure temperature too high: Reduce conveyor temperature; increase dwell time instead.
• Flash cure too intense: Reduce flash cure temperature; use a gentler, longer flash.
• Test more garment brands: Some polyester garment dyes migrate worse than others. Find a brand with better dye stability for bleed-sensitive work.

Problem 5: Underbase Cracking After Washing

Symptoms: Underbase peels or cracks after the first few washes, even though top colors are intact.

Causes and fixes:

• Underbase undercured: The thick underbase ink deposit requires longer cure time than thinner top color layers. Ensure the full depth of the ink deposit reaches cure temperature. Use a temperature probe inserted into the ink layer, not just a surface temperature check.
• Underbase ink too rigid for stretch fabric: On athletic and stretch fabrics, use a stretch-additive or flexible underbase formulation.
• Ink deposit too thick: Very thick underbase layers can crack at flex points. Use double-stroke technique with slightly less ink volume rather than a single extremely heavy deposit.

Problem 6: Underbase Printing Unevenly / Patchy Coverage

Symptoms: Underbase shows light and dark patches; some areas of the design have good coverage, others show the dark garment through.

Causes and fixes:

• Mesh tension low: Uneven snap-off causes variable ink deposit across the print area. Check and correct tension.
• Squeegee pressure inconsistent: On manual presses, squeegee pressure varies with operator technique. Practice consistent stroke technique.
• Garment not flat on platen: Wrinkled or misloaded garment creates uneven print surface. Ensure garment is smoothly loaded and pallet adhesive is fresh.
• Screen sagging in center: Large screens with low tension sag at center, reducing off-contact and causing heavy center / light edge. Re-tension or use a smaller screen.

Common Mistakes with Underbase White

1. Using the same white ink for underbase and highlight white. Underbase inks are formulated for high opacity and flash cure compatibility. Highlight whites are often softer and more transparent. Using the wrong white for each application gives suboptimal results.

2. Not choking the underbase. Printing underbase at exactly the same size as the top colors guarantees a white halo whenever registration is less than perfect. Choke as standard practice.

3. Using the same mesh for underbase as for top colors. Underbase requires heavy ink deposit — 55–77 TPI. Top colors require finer mesh for detail — 110–160 TPI. Always use a coarser mesh for the underbase screen.

4. Overflashing the underbase. A glassy, over-cured underbase surface reduces top color adhesion and can cause delamination in washing. The underbase surface should be dry-to-touch but not fully cured.

5. Ignoring polyester dye migration. Using standard white on polyester and then being surprised by color shift is an avoidable problem. Always identify the garment fiber content before selecting underbase ink.

6. Not testing underbase coverage before a production run. Print a single underbase-only shirt and hold it up to a light source. Pinholes, light coverage areas, and edge definition problems are clearly visible on a single test print — before committing to a full production run.

7. Designing the underbase as a simple white fill of the artwork silhouette. A well-constructed underbase is a separate, carefully designed layer — choked, with shadow areas adjusted for halftone underbase where appropriate. The extra 30 minutes of underbase design time produces significantly better final results.

Summary

The white underbase is not background work — it is the foundation of every dark garment screen print. The time invested in correct underbase design (choke, selective coverage, halftone underbase for photographic work), correct ink selection (high-opacity plastisol, low-bleed formulas for polyester), correct mesh (55–77 TPI), and correct flash curing (dry-to-touch, not over-cured) determines the vibrancy, accuracy, and durability of everything printed on top of it.

Treat the underbase as a critical color station, not a preparation step, and your dark garment printing will improve immediately.

Dragonfly Colors includes underbase design as part of every dark garment color separation service. Contact us for professional prepress support.