Wood and fibreglass seem like something of a perfect match. Fibreglass resin is designed to soak into a material matrix to strengthen and seal it, be that matrix of glass fibre or plant cellulose. Fibreglass resin can seal wood to form a shell that protects the wood, and provides a smooth painting surface. It also fills in any imperfections. Glassing over wood requires a few precautions and techniques to ensure uniformity, strength and bonding.
Turn the heat in your workshop up until the room reaches about 32.2 degrees C (90 degrees F), and allow it to maintain that temperature for 20 minutes before beginning the fibreglass procedure. Place the resin in front of your heater vent to bring it up to about 29.4 to 32.2 degrees C (85 to 90 degrees F). Heating the room in this way will open the wood pores to prepare them for fibreglass application.
Mix the resin and hardener according to manufacturer recommendations. Do not mix any more than one cup at a time, as the resin/hardener reaction will create its own heat and speed curing. Pour resin into disposable roller tray. Soak the foam-head roller with resin very thoroughly, but roll it out in the pan afterward to remove excess.
Roll the resin-soaked roller directly onto the wood surface to seal it. Use very slow, even, diagonal strokes to apply the resin without causing it to foam up. Cover the entire wood surface, and then open your shop door to quickly vent the hot air. This will cause the wood pores to shrink while the resin sets up, sucking the resin into the wood and preventing bubble-causing "out-gassing" from the wood. Allow the resin to set up completely until it is completely hard.
Sand the entire surface with 120-grit sandpaper to help the next coat of fibreglass stick.
Repeat Steps 1 and 2. Cut your fibreglass mat into 15 cm (6 inch) strips and lay them on the surface. Run the resin-soaked roller over the fibreglass mat, soaking it thoroughly but not so much that the resin builds up and runs. You might be tempted to simply pour the resin over the mat and spread it, but do not; the glass mat will float to the top of the resin and show through to the finished surface.
Soak your foam brush in resin and very gently run it over the fibreglassed surface to rid it of any bubbles. Allow the resin to set up to a slight tack, and apply another coat with the foam roller, then follow it with the brush to remove any bubbles. Allow the fibreglass to harden fully -- at least two hours to ensure that it is completely set.
Sand the surface with 500-grit sandpaper, and then 1000-grit sandpaper. Wet-sand the finished resin with 1000-grit sandpaper to leave a beautiful, transparent surface that will bring out the grain of the wood and keep it from harm.
If you are planning to fibreglass an old wooden boat to restore it, bear in mind that the fibreglass resin will seal into the wood any moisture, bacteria or mould it contains. These things will eventually cause the wood to rot inside the fibreglass shell, leaving it visually acceptable but structurally unsound.