Nict to know

Which Abrasive Paper?

Following sanding, the surface must be thoroughly cleaned again since sanding dust can adhere to it more firmly than one would expect. Rinsing with water, or removal with compressed air is often only partially effective. A thorough removal of sanding dust can only be achieved through a combination of brushing and rinsing – hose in one hand, brush in the other. Re-rinse with ample fresh water.

Before coating can commence the surface must be absolutely dry.

  Dry Sanding Paper
Wet Sanding Paper
(silicon carbide)
Sanding colour remnants following chemical stripping  40 – 80    
Sanding of filler  60 – 100    
Sanding of primer or antifouling    80 – 120  
Sanding of previously lacquered surfaces or gel coatings prior to a re-coating  120 – 200    180 –240  
Prior to minor repairs to a lacquer coating    240 – 400  
Sanding prior to final lacquer coating    300 – 400  
Prior to the polishing of 2-component lacquers    – 1200  

Drying time

Drying time is highly temperature dependent – whereby temperature refers to surface temperature during the drying process. Low temperature delays drying, higher temperature speeds up drying. Every 10 °C increase in temperature will have a significant effect on drying time. The chemical curing of 2-component products usually requires a minimum curing temperature of +10 °C since the curing process draws to a halt at lower temperatures.

A second important factor in the drying process is the relative humidity. The higher the humidity, the longer drying will take. Relative humidity above 80 % is critical (surfaces can become matt, with the dew point being reached in localised points). Furthermore, air circulation and sunlight should also be taken into consideration. Since solvent vapours are heavier than air, when working in a closed room ensure good air circulation to avoid the slow drying and matt surfaces which vapours can cause. Otherwise, please refer to „Possible Dangers and Safety Tips“.

Interval between coats

For all 1- and 2-component products both a minimum and maximum must be closely adhered to. The interval between coats is applicable not only to re-coating of the product with itself, and also to coating with further products. An “early” application of a further coat leads to solvent enclosure, resulting in deficient curing. In turn this will cause blister formation when under water loading. “Late” application of the following coat hinders bonding with the new coat since, in the case of 2-component products, the material is already totally cured. Where the maximum interval between coats has been exceeded it is essential, also in the case of 1-component products, that the surface be matt-sanded before the new coat is applied (ensuring the mechanical fixing of the newly applied coat).

Dew and frost

Dew and frost are well known. Their formation is due to the different saturation levels of water in air at different temperatures (a cubic metre of air can hold approx. 1 g of water at - 18 °C, 5 g at 0 °C, 20 g at + 23 °C and around 30 g at + 30 °C). When the maximum amount of water which can be held by the air at a given temperature is reached, the relative humidity is said to be 100 %. Cooler surfaces can become covered in a dew film, which can be invisible, brought about, for example, through wind, evening cooling, tanks being filled, drying rain, or through drying paint. For this reason it is usual practice only to carry out coating work when the temperature is at least 3 °C above the dew point (or at a maximum relative humidity of 80 %).


„Real“ osmosis is to be understood as contact surface blisters formed between the gel coat and laminate, or within the laminate, containing a liquid which is reactive to acids.

In assessing the extent of the damage the following factors are important:

  • Quantity, size, local distribution and pattern of the blisters.
  • Affected areas (as a percentage), delamination, and whether mechanical stability is still ensured.

The following have come to be recognised as causes of „real“ osmosis:

  • Gel coat is too thin or microporous, or too highly filled
  • Air voids
  • Insufficient hardening or preparation
  • Hydrolysis
  • Ageing
  • Natural diffusion of water
  • Damage due to chlorinated hydrocarbons such as methylene chloride, dichloromethane and trichloroethane.

The product labelling will indicate whether any of the substances mentioned above, which would be damaging to GFRP materials, are contained in the product. To avoid a re-occurrence of “real” osmosis, yachts and boats, whether new or in use, should be preserved with a preventative coating of epoxy primer of the recommended film thickness.