Sand GRP to the same height


The grinding of fiberglass GRP ranges from the slight sharpening of an undamaged surface of gelcoat to the deep grinding free of GRP itself. In addition to the function of sanding out damage, the grinding in of repaired areas and surfaces is necessary to prepare the new paint job.

Osmosis at the underwater boat

In the case of the GRP of an older boat, the overwater and underwater vessel must be distinguished. The particular physical load also has an effect on the proper grinding of GRP.

Similar to the subsequent painting of the GRP, the risk of osmosis in the underwater boat must be minimized by the greatest possible avoidance of air bubbles. Therefore, according to the result of the damage analysis, a "bottom grinding" up to the first laminating level of the GRP is recommended.

Level the layer depth

In addition to large-scale grinding, often partial sanding of damaged areas is necessary when repairing GRP. After exposing osmotic bubbles should be ground out.

To ensure effective grinding when repairing GRP, knowledge about the structure of the material helps. In principle, the goal should be the reconstruction of a homogeneous body with adequate layering technique during deeper grinding work, which extends through the entire gelcoat layer into the GRP.

How to grind GRP

  • Abrasives with grain sizes 80, 400, 800 and 2000
  • Grease-dissolving cleaning agent
  • wire brush
  • orbital sander
  • Delta sander
  • Suction device when working in closed rooms
  • Respiratory and eye protection
  • gloves
  • Microfiber cleaning cloth

1. Determine the grinding depth

First, you should use a wire brush or hand sandpaper to scrape and scratch dirty spots or potential damage to check the texture and depth. In the case of corrugated or grooved surfaces, the removal must at least extend into the tire or scored valleys.

2. Pre-cleaning

Clean the surface or areas you want or need to grind with degreasing cleaner. Any contamination sets your abrasives.

3. Increase grain size

Begin the sanding process by using the coarsest 80 grit and observe the sanding depth. They recognize the different depths of the layers of GRP by color changes.

4. Abrasive grinding

Whenever possible, achieve a consistent sanding depth across the entire fiberglass surface, including repaired and puttied areas.

Tips & Tricks

Note the rapid "clogging" of the abrasive and ensure a correspondingly large supply, especially in the higher grades.


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