1. Primer Coating
An aqueous coating specifically formulated for use as a primer is generally used. It can be applied in-line during the offset printing phase of the job.
The primer/ink film combination must dry before applying a UV coating otherwise the UV coating's gloss and resistance properties will decrease over time if the aqueous primer has not properly dried.

2. Ink/Water Balance
Compared to oxidizing offset inks, UV inks have poor water pickup characteristics and are more likely to emulsify. Excess water carried on press will interfere with the transfer and adhesion of UV inks. Therefore, ink/water balance is even more critical with UV inks than with oxidizing inks.

B. Sheet Distortion
Introduction
Free radical UV coatings cure very rapidly. They can shrink three-dimensionally from 5-20%, depending on their formulations. This causes internal stresses in the polymer structure and lowers coating adhesion. UV coatings must be formulated to minimize this shrinkage and to maximize adhesion.
Because of the paper's polymer structure and the heat generated during UV irradiation, YUPO can expand in the grain direction and contract in the cross-grain direction during UV printing. Therefore, it's important to minimize the variations in UV light energy output and the build-up of excessive heat to maintain the paper's dimensional stability and avoid registration problems.

1. Tips to Maintain Registration
With this potential for sheet distortion, UV printing is not recommended for basis weights below 78 lb. text (FPG 150); heavier basis weights are not as susceptible to temperature distortion. Since their dimensional change is dependent on grain direction, all sheets must be run in the same grain direction to avoid registration problems. For multiple runs or two-sided printing, sheet temperature must be constant with the initial press run. The general recommendation is to print UV jobs on YUPO in a single pass.
Heat build-up in the delivery pile may also cause sheet distortion, particularly in the center of the load. If feasible, blow cool air on the load to reduce pile temperature. Load temperature may also be reduced by running smaller lifts.

2. Inter-Deck Curing
Many UV presses can inter-deck cure UV. Several advantages of this method include: inks are dry-trapped during multi-color printing; ink adhesion off the press is improved due to repeated exposures to UV light and a higher effective cure rate.
However, when printing YUPO, inconsistencies in UV energy output between curing units and the progressive build-up of heat down the press can cause sheet distortion and registration problems. The decision to inter-deck cure will depend largely on registration requirements of the print job and the ability to consistently control energy output between printing units.

C. Compatibility with Oxidizing Inks
1. Wet Trapping Not Advised
Wet trapping free-radical UV coatings is not advised when printing with fully oxidizing inks. If UV coatings are applied over these inks while wet, the oxygen, critical for the ink drying process, retards the curing process. As a result, the desired coating properties such as gloss and rub resistance may not reach optimum levels. Dry-back, or reduction of coating gloss, will also occur as the wet ink film pulls in the drying UV coating. Dry-back worsens with heavier ink coverages.
2. Risk to Proper Drying
Inks can also take significantly longer to dry when UV coatings are applied over wet oxidizing ink film. Since YUPO has poor air permeability, little oxygen can pass through the backside of the sheet to assist drying the ink film on top. Therefore, oxygen must be supplied to the ink film from above. If a UV coating is applied over the ink, the highly cross-linked film will slow down or stop the air (oxygen) flow required for inks to dry.
3. Recommended Drying Times
Allow sufficient time for the oxidizing ink film and primer coating to dry completely before overprinting with a UV coating—about 72 hours under normal pressroom conditions. If the job requires printing on both sides, print each side within 24 hours to reduce the possibility of gas ghosting. Then allow the job to dry 72 hours before coating.

D. Off-Line Coating
1. Minimize Spray Powder When Overcoating
Since oxidizing inks and primer coating must dry prior to coating, YUPO printed by conventional offset requires off-line UV coating. Therefore, it's extremely important to coat over as little offset spray powder as possible. Trapping too much spray powder beneath the UV coating will give the printed piece a rough, grainy feel.
2. Removing Excess Spray Powder
To help remove excess spray powder, dust the press sheets by running them through a blank printing unit prior to UV coating. Static has a tendency to build up on YUPO when run through a press, particularly with heavy ink coverage. When dusting YUPO, run the press sheets through the printing units with just enough water to prevent static build-up. Avoid using too much water; it can impair the coating's adhesion on the stock. Excess water not dried from the printing surface will not be absorbed by YUPO, and it will interfere with the coating's ability to bond. It may also soften the ink and cause blocking in the loads.
3. Monitor Infrared Heat Output
UV lamps generate infrared heat as a by-product. Excessive heat may build up in the delivery pile, raising the load temperature to a level that will cause YUPO (particularly lighter basis weights) to distort. Delivery pile temperatures should be monitored and kept below levels that can cause this distortion. UV coating is not recommended for basis weights below 78 lb. text (FPG 150).

E. Ink Requirements
1. Avoid Inks with Slip Improving Waxes
Do not use inks containing slip-improving waxes; the infrared heat generated during UV curing will melt the waxes, resulting in poor coating adhesion. If the coating crawls on top of the ink surface or flakes off check the ink formulation for waxes.
Note: Simply specifying a wax-free ink will not ensure acceptable results. Other ink properties are crucial for successful UV coating. Consult your ink supplier for appropriate recommendations.
2. Additional Time Required for Darker Inks
Ink colors can also affect the curing rate of UV coatings. Darker ink colors absorb more UV light as it passes through the coating. The initial cure level will be lower with darker colors, and additional time will be required for the coating properties to develop.
3. Avoid UV Light Sensitive Inks
Avoid ink colors formulated with UV light sensitive pigments. They will undergo a significant color shift when UV coated. Several pigments susceptible to shifting are: Reflex Blue; Rhodamines; Purple; Violet; some Fluorescents.
4. Metallic inks, not recommended for use on YUPO, are also difficult to UV coat. Avoid full strength and tint inks using these colors in their formulations.