Introduction
Vacuum infusion is increasingly accepted by the yacht building industry. The positive effects this process present are for example better mechanical properties with respect to regular hand  lay-up techniques and lower hazardous styrene emissions. However, in one aspect vacuum infusion can behave poorly when compared to the hand lay-up: surface quality.
Higher fibre volume fractions and the volume shrinkage of the styrene based resins lead to more fibre print-through on the surface of the laminates. Resin suppliers are currently investigating how to improve this phenomenon of vacuum infusion.
Several methods to conceal the fibre print-through are investigated. The effectiveness of fibre print-through blockers like hand lay-up chopped strand mat (CSM) plies, barrier coats and surface veils was studied and described briefly on this webpage.
 

Surface quality
The surface quality of a boat or car plays a major role in how customers perceive the product’s quality. The ideal look would be mirror-like, that is, a surface with high gloss and perfectly smooth. In the past the appearance was described with a variety of terms such as glossy, brilliant or dull, orange peel, smooth etc. In other words, each car manufacturer defined its own standard as to specifying an acceptable finish. The result was a subjective opinion about the surface. However, what might be considered a good surface by one person might not be good to another.
 
Measuring surface quality
Nowadays all major car manufacturers use a wave-scan as standard equipment to quantify appearance. A wave-scan assigns an objective value to the appearance. The appearance of a coated product is determined by two phenomena: “Image clarity” and “Distinction of Image” (DOI). The wave-scan has now also been used to objectively determine the surface quality of yacht structures.
 
Image Clarity
Image clarity is a property, sometimes referred to as “orange peel”, is caused by the macroscopic surface roughness patterns of the product (wavy structures between 0.1 and 30 mm in structure size). This imperfection reflects light in various directions and only the elements reflecting light in the direction of our eyes are perceived as light areas.
 
Distinction of Image
Distinction of Image (DOI) is a measurement of the brilliance of a reflected surface. It determines the dullness/brightness and sharpness of the reflection.

BYK Gardner Wave-scan DOI
The wave-scan DOI of BYK Gardner is a small device (figure 1) that – when rolled over the surface you want to scan – measures the orange peel and DOI of the surface. The measured signal is divided into several ranges:

A measured signal close to 0 is an almost perfectly smooth surface. The higher the value gets, the rougher the surface. 
 
Fig.1  BYK gardner wave-scane DOI

Resolution of the human eye
Perceiving wavelengths between 0.1 and 30 mm by the human eye depend on the distance to the object. Very fine structures (structures in the du, Wa and Wb ranges) can only (if at all) be perceived as wavy patterns at very close distances and from a longer distance they are perceived as dullness and brilliance (DOI). These values are highly influenced by what kind of gelcoat is used, since different coloured gelcoats have different gloss.
The values in the Wc, Wd and We ranges are best seen from distances above 1 meter and the lower the values in these ranges, the smoother the surface seems to be.
Since customers usually look at a yacht from a distance of at least 1 meter, the Wc, Wd and We ranges are the most important to judge surface quality.
 
Experimental
In the laboratory of Lightweight Structures B.V. a series of vacuum infusion laminates were made on flat glass plates using a conventional release agent, several glass reinforcements and fibre print-through blockers. Neogel ECO 9375 (from DSM Composite Resins) is a brush-grade white gelcoat commonly used in the boat industry. Synolite 1967-G-3 (from DSM Composite Resins) is a DCPD based unsaturated polyester resin. DCPD based resins combine low viscosity with low shrinkage making them excellent matrix materials for various composites. Synolite 1967-G-3 is recommended for vacuum infusion of boats and can meet Det Norske Veritas rules, grade 2 for boat building.

First experiment: no print-through blockers
The laminate without any print-through blocker consisted of the reinforcement material S375/G500/S375 Multimat (1250 g/m²) directly applied on top of the gelcoat. The laminate was infused with Synolite 1967. A similar replica of the laminate was infused with an orthophthalic based resin, Synolite 2503.
To visually determine the surface quality of the laminates pictures were taken of the reflection of a spotlight on the surface. The print-through can clearly be seen on the pictures in figure 2. Also, comparing the two laminates shows that the surface quality of the laminate infused with the DCPD resin Synolite 1967 is better than the one infused with the orthophthalic resin Synolite 2503.

Synolite 1967	Synolite 2503

Fig. 2 Reflections of a spotlight on laminate surfaces
 
The results from the wave-scan DOI are shown in figure 3. The values in the du, Wa and Wb ranges are almost exactly the same, because these wavelengths are mostly influenced by the gelcoat. Looking at the values from the Wc, Wd and We ranges it is clearly shown that the DCPD resin yields a slightly smoother surface.
 

Fig. 3 Spectrum of the laminates infused with the two different resins

Next experiments: laminates with one ply hand lay-up CSM, Spraycore and Soric
A barrier coat is a material that is applied directly after the gelcoat to protect the laminate. When the fibre layers and the matrix resin are applied, the barrier coat blocks the heat of the reaction and protects the gelcoat against any distortions. Three types of barrier were used:

The laminates consisted of a number of combinations of these barrier coatings and 500/S375 Multimat (1250 g/m²), infused with Synolite 1967.
The Soric veil showed superior qualities in masking the print-through compared to the Spraycore. The combination of both Spraycore and Soric along with the CSM ply resulted in a very smooth and print-through free surface. In fact, this combination of print-through barrier  material has a reflection almost equal to that obtained in a mirror (comparing smoothness and not brilliance), see figure 4.
  

Figure 4 Laminates with Multimat and Spraycore, Soric SF and/or CSM print-through barriers

The results from the wave-scan DOI are shown in figure 5. The change of barriers led to further improvements in the Wa, Wb, Wc and Wd ranges. Clearly the high volume fraction of CSM lends a helping hand in masking the surface defects caused by the resin shrinkage.


Figure 5 Spectrum of laminates with Spraycore, Soric and CSM as print-through barriers and Multimat reinforcement (red, bue and yellow graphs; the white ones have another glass reinforcement)

By choosing the correct print-through blockers it is possible to improve the surface quality to the point where it is nearly equal to the surface quality of the mould used.

Adding a barrier coat or a surface veil, between the gelcoat and the reinforcement, will efficiently stop fibre print-through in the gelcoat. This will also lower the values in the spectrum ranges down to values in the vicinity of the mould’s. If the mould were completely smooth – like the glass plate used to infuse the laminates – this would result in a very smooth surface. As it is now, the best smoothness that can be achieved is equivalent with the smoothness of the mould.