Reproducing the details of an old wooden boat hull in fibreglass is a straightforward procedure, but it requires quite a bit of preparation. In order to work cleanly with fibreglass, the boat needs to be free of loose paint and rotted or soft wood needs to be hardened. The hull also needs to be adequately sealed and treated with the right release products before the fibreglass is applied, or the mould may wind up flawed or damaged.
Set the hull on its gunwales on a pair of saw horses. If the hull is too wide to sit securely on a single sawhorse, lay planks against pairs of saw horses and set the hull on the planks.
Remove all loose paint on the hull with steel wool or a wire brush. If the paint is generally chipping and flaking, remove all of it.
Harden any soft, pulpy or rotted wood by applying wood hardener. Apply enough hardener with a paint brush so that the soft wood is completely saturated, then allow it to cure according to manufacturer's directions. Sand any rough spots once the hardener has cured.
Fill in any gouges or missing chunks of the hull with wood putty. Apply the wood putty with a putty knife and smooth it as close to the original contours of the hull as you can. Once the putty has set, sand it smooth.
Spray the surface to be moulded with acrylic spray sealer. Be sure to seal all external details.
Apply wax mould release to the exterior of the hull with a cloth. Be sure to coat all surfaces that will be fiberglassed. Buff the wax to a smooth sheen with a clean cloth.
Apply liquid mould release to the hull according the manufacturer's directions. Be sure to adequately coat all surfaces that will be fiberglassed.
Apply fibreglass putty to all details on the hull, such as the corners of lap strakes, the undersides of the gunwales and the contours of the keels.
Mix two part epoxy in a clean bucket, according to manufacturer's directions. Using a paint roller, apply a coat of epoxy to the hull.
Saturate fibreglass matting in epoxy and apply it to the exterior of the hull. Start from the keel and work down. The matting should run horizontally down the hull, and completely cover the exterior, including any fibreglass putty that has been applied. Allow the epoxy to harden.
Apply at least two more layers of fibreglass matting, using the same procedure. After the last layer, allow the epoxy to reach full strength, according to manufacturer's instructions, which is often several days.
Lift the fibreglass mould away from the hull. Remove any debris that might be clinging to the inside of the mould, if possible.
This process results in a female mould where you apply fibreglass putty and batting to the inside, to create a hull that will duplicate the shape and details of the original hull exterior. If your hull has protrusions or contours that will prevent it from releasing from a one-piece mould, you'll need to make a two-piece mould. You do this by building up a thin wedge of modelling clay down the centre line of the hull. The ridge needs to be at least as deep as the planned thickness of your mould. Seal this ridge and apply mould releases to it just as you do the rest of the hull. When you lay on fibreglass matting, make sure not to cover the top of the ridge. When you demold, the fibreglass will pull away in two parts.
Fibreglass epoxy products emit harmful vapours so this work should be done outside, or in an area with excellent ventilation. Avoid applying epoxy resin in areas where overhanging trees or other vegetation can drop debris on the hull. These can become trapped in the curing resin and be very difficult to remove.