Fiberglass Fillers: How to Protect Your Hull and Extend Its Life

Throughout the course of boating, it’s natural to expect some scratches, dings and even dents. Fiberglass is notorious for its resistance to surface damage, but sometimes even the best boater can scratch the bottom or run into docking hardware. Luckily, there are many fiberglass repair systems out there known as fillers that can patch up any rough areas and make your boat look new again.

When repairing a fiberglass hull, it’s very important to use a filler designed for fiberglass. Fiberglass typically has a gel-coat, and unless you’ve painted it, you’ll need to ensure your filler medium will bond to the fiberglass hull substrate. Any filler you buy will have a designation listed on its label, stating its multi-purpose (wood, aluminum, etc.) or for a specific substrate (i.e. fiberglass).

Below the waterline of your boat, you must always use an epoxy-based filler because epoxy is typically oil-based. This repels water and ensures an absolute watertight repair.

When selecting the kind of filler for your fiberglass hull, you must also consider the size of the area you want to patch. Any boat will eventually develop small imperfections in its hull. If you’re only patching small pinholes or tiny scratches, your best is to use a marine putty. These putties come in smaller tubes and are easier to handle than fillers. They also come pre-mixed, cutting down on application time.

Fiberglass FillersIf you have a larger area to repair and you’re a do-it-yourselfer, you should always go with a fiberglass filler or a fiberglass resin with hardener. The first step to repairing is prepping the area to be repaired with sanding and cleaning. You can’t slap a layer of filler over the area without properly prepping the surface. If you do, you won’t have good adhesion and you can end up trapping water against the hull, which will cause more damage in the long run and blistering.

After you have prepped the area, you apply filler much in the same way you would repair a hole in a wall, with a putty knife and alternating levels of sanding to ensure a smooth finish before you paint over the area. Always use primer and the correct kind of paint that matches your existing job. After all, no one wants a mismatched hull and using the same paint will provide maximum uniform protection of your hull.

If you have an exceptionally large area to patch, such as a large dent, it may be best to consult with a professional boat hull repairer. Large areas of repairs will often require the use of fiberglass cloth or mats, which are typically only recommended for professional use or for those who have done major repairs before. If you try to tackle a big job without prior experience, you may end up doing a poor job and actually hurting the integrity of your hull over time. Or worse, you could repair your hull only to find that the filler doesn’t hold up and flakes off.

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