Hull repairs

Blanking off a through-hull fitting

Based on articles submitted by Jay Blackburn and sCorribee, August 2009

Fitting new, and moving or removing existing fittings in a boat may involve blanking off an existing through-hull aperture. Work above the waterline calls for a good quality finish, while work below the water line demands good structural integrity. The notes below refer to a repair to a 38mm (1 1/2” ) diameter hole – a typical ‘Corribee-sized’ cockpit drain or sea toilet fitting. You may need to scale the dimensions up or down for larger or smaller fittings.

1. Remove the existing fitting. A large adjustable spanner or pipe wrench is useful, and you may need a bar to insert into the fitting itself to prevent it turning when undoing the back nut. As a last resort, careful use of an angle grinder or a Dremel will be needed, (obviously destroying the fitting in the process).

2. Clean up the hole. Remove any antifouling paint or other finish on both sides of the hole. Use a coarse, half-round file to chamfer the edges of the hole on both sides (see Fig 1). Doing this will provide a mechanical key for the repair. Also you need to thoroughly abrade the area around the hole on the inside for at least 75mm beyond the edge of the hole, so you expose the glass fibres in the original layup.


3. Remove any oil, grease, wax etc. Use acetone to thoroughly clean the existing grp, to ensure that you create a sound chemical bond between the original layup and the new composite.

4. Prepare the cloth. Using woven glass cloth (600 g/sq m) cut out a series of circle-shaped patches, starting with 3 or 4 the same size as the hole, followed by a further 6 or so which gradually increase in size, the last one being the same size as the area of hull which is cleaned and abraded. If you are working to the the suggested sizes shown above, the last circle will be approximately 188mm (7 1/2”) diameter (2 x 75, plus 38mm).

5. Mix some epoxy resin – SP Systems 106 or the West Systems equivalent. Digital kitchen scales are useful to obtain the correct mix ratio. Start with a mix of around 100 grams, as you can always mix more as needed. Brush some of the resin around the hole inside and out.

6. Final preparation of the hull. Working on the outside, apply several strips of parcel tape over the hole as smoothly as possible. Any wrinkles will appear on the finished repair and will need filling, so it’s best to avoid them in the first place.

7. Prepare the lay-up. See Fig 2. Tape a piece of polythene sheet onto a board. Starting with the smallest glass cloth circle first, put the first one down on the polythene and wet it out thoroughly with a little resin. Add the subseqent circles, wetting each one in turn with the resin. You will finish up with a shallow mound of wetted-out cloth.


8. Apply the repair. Lift the complete lay-up on its’ polythene sheet off the working board and apply it resin side down to the hole. Carefully remove the polythene and gently (so you don’t press too hard on the parcel tape underneath!) smooth it into place, making sure there are no voids or air bubbles.

9. Finish off. If you have any resin left over, apply a final coat over the whole repair on the inside to ensure there are no stray fibres sticking up. After 24 hours or so, when the repair has completely cured, the parcel tape can be removed and the outside surface of the repair can be rubbed down, filled, primed and then painted. Filling minor imperfections can be done with car body-filler, and International Primocon is suitable as a primer before antifouling.

Some notes:

Epoxy resin will form a strong bond with existing (ie fully cured) polyester grp lay-ups. Polyester resin, on the other hand, does not bond to fully cured polyester grp satisfactorally. It is therefore strongly recommended that you use epoxy resin for all repairs, but especially any that are below the waterline, or are subjected to any load. Polyester resin repairs should only be used on cosmetic, non-structural repairs, eg filling gel-coat chips or hairline gel-coat cracks.

Correctly mixed epoxy should start to cure within an hour or so, but will need several hours before it reaches full hardness. The temperature should be above around 15 ºC, otherwise there is a danger of an incomplete cure. You might need a fan heater or hair dryer in winter to help in obtaining a satisfactory cure.

Methylated spirits is a useful solvent for epoxy resin and can be used to clean tools and brushes.

You may need to thicken the epoxy resin for some repair processes. This can be done with a number of additives, including:

  • Microfibres (especially useful for bonding, and produces a hard, tough epoxy – quite difficult to rub down with glasspaper when cured).
  • Colloidal silica (suitable for bonding, gap filling and filleting. Increases the viscosity of the resin, so less likely to drain out on vertical and overhead repairs).
  • Microsphere (hollow glass spheres, will produce a filler suitable for fairing, easy to sand, but must be overcoated with further coats of epoxy if used below the waterline).
  • Other fillers are also available – have a look at the manufacturers web sites for details.

‘Wetting out’ is the term used to describe the process of saturating the glass cloth with resin. As the cloth becomes saturated it becomes semi-transparent, so it is easy to check that the cloth is fully wetted. When preparing the lay-up, the normal working practice is to apply the layers of cloth wet-on-wet so there is a full bond between the layers.

Take some sensible safety precautions – in particular avoid breathing the dust from antifouling paint and the fibres from glass cloth. Use vinyl gloves to protect your skin from the epoxy resin (some people can become sensitised to it, resulting in dermatitis). Avoid inhaling the grp (or any other) dust from angle grinders. Epoxy resin packs usually include extensive health and safety information.

List of materials:

  • SP Systems 106 resin and hardener (or the West Systems equivalent).
  • Methylated spirits
  • 600 gsm woven glass cloth
  • Acetone
  • Coarse glass paper – eg 60 grit aluminium oxide paper
  • Parcel tape

List of tools:

  • Angle grinder, spanner, adjustable grips etc (to remove existing fitting)
  • Paint scraper (to remove anti-fouling paint)
  • Rag
  • Coarse half-round file
  • Work board
  • Scissors
  • Polythene sheet (heavy duty 500 gauge is recommended)
  • Mixing dishes (polythene is best)
  • Mixing sticks (like large lollipop sticks, used for mixing and wetting out resin)
  • Cheap disposable paint brushes (for wetting out resin)

Useful links:


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