Trade Secrets: TRADE SECRET: Fibreglass boat repair: repairing laminates using vacuum bagging
When faced with a laminate fibreglass boat repair, there are several methods which could prove effective in achieving a decent repair. But nothing gets you quite as close to the original material as vacuum bagging. Although interpreted as a more advanced method, even the least experienced epoxy DIYer can perform a repair using vacuum bagging, with the right process.
What is vacuum bagging?
Vacuum bagging is an extension of the wet lay-up process, where pressure is applied to the laminate – once laid-up – to maximise the consolidation of the fabric. It’s a practical clamping method for large scale and very small scale applications, from product manufacturing to backyard building and hobby projects – which makes it perfect for DIY repairs.
Why use vacuum bagging?
For most fiberglass boat repairs, vacuum bagging isn’t strictly necessary but for highly stressed, lightly built composites, using vacuum bagging techniques to laminate new fabric into an excavated damaged area is an extremely effective way to achieve a high quality repair, with maximum consolidation that should be as strong as the existing material.
What’s the process?
- Prepare to repair
Using a grinder with 40-80-grit paper, grind out the damaged area into a circular or oval shape. You should remove any delaminated laminate, exposing the solid undamaged laminate underneath. Then, bevel the edges of the cavity to a 12 to 1 angle. For vacuum bagging to be effective, the area needs to be free of any airways, so seal the back of the opening to provide an airtight envelope.
Trim some fibreglass cloth to the same shape as the excavated repair area. You’ll need enough so that the combined thickness of the layers is slightly thinner than the original laminate. Trim the first piece so that it’s slightly smaller than the outside of the bevelled edge. Each of the remaining pieces should be cut slightly smaller than the preceding piece with the last piece the same size as the bottom of the cavity at the inside of the bevel.
Next prepare the vacuum bagging materials. Cut your release fabric, perforated film and breather slightly larger than the repair area. Then cut the vacuum bag several inches larger than the repair area and apply some mastic sealant several inches outside the perimeter of the repair area.
- Wet-out process
Now apply the wet-out layers of cloth. Start with the largest layer and then continue with the progressively smaller layers centred in the repair area, wetting-out each layer of cloth on a plastic covered table first, then smoothing each layer into place on the repair.
To ensure a better finish, remove any air bubbles and excess epoxy with an 808 Plastic Squeegee. Use this to squeegee the layer of release fabric over the layers of cloth – again removing any trapped air and excess epoxy. Place the perforated film and breather material over the lay-up and seal the vacuum bag to the mastic.
- Vacuum bagging
Be sure to attach the vacuum port off to the side of the repair to prevent dimpling the repair. Then, turn on the vacuum pump and attach the vacuum gauge. After the vacuum has stabilised, moderate heat from a heat lamp or portable dry heater may be applied to the lay-up to speed up the cure.
Allow the lay-up enough time to cure thoroughly. Then remove the bag, breather, perforated film and release fabric.
Next, grind any high spot or bumps and fill any low areas with a thick mixture of WEST SYSTEM epoxy and 407 Low Density Filler and allow the mixture to cure. To seal the repair, sand the area thoroughly and apply two coats of WEST SYSTEM epoxy to finish.
For a full guide to vacuum bagging applications, including health and safety advice, expert techniques, as well as further boat building and maintenance publications, visit West System International: https://wessexresins.co.uk/support/publications-expert-guidance-for-specialist-epoxy-applications