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Fiberglass tubs are thin, flexible, and easy to chip or crack if a bit grabs the surface. For most clean holes, a sharp, ordinary twist bit is enough; the bigger risks are using hammer mode, pushing too hard, or letting the bit wander. Choose the bit for the hole size and any fitting you’re installing, then drill slowly with the tub supported from behind where possible.
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Choose the bit for the hole
For small pilot holes and many holes up to about 1/4 inch, use a sharp high-speed steel (HSS) twist bit. It is inexpensive and cuts fiberglass well when used at low speed with light pressure. A standard HSS bit is a sensible choice for a one-off repair; buying a specialty bit is not necessary for every tub job.
For larger holes—such as openings for plumbing fittings—use a hole saw sized to the fitting’s required clearance, not just its outside trim. A fine-tooth carbide-grit hole saw is often a better choice than a coarse woodworking hole saw because it abrades the laminate instead of catching it with large teeth. Confirm the manufacturer allows use on fiberglass, and check the fitting template before cutting. If the opening is small and a hole saw is unavailable, a step bit may work in thin laminate, but stop at the required diameter and avoid forcing it.
Use a fresh bit if the one in your drawer is dull, rusty, or damaged. A blunt edge needs more pressure, which increases heat and the chance of cracking or splintering. A cobalt bit is not a requirement: it is designed mainly for hard metals and offers little practical benefit for ordinary fiberglass work.
Compare HSS twist drill bit sets if you need a basic range of small sizes. For a larger, round opening, compare carbide-grit hole saws for fiberglass and match the diameter to the fitting instructions.
Bit types at a glance
| Bit type | Best use | Trade-off |
|---|---|---|
| HSS twist bit | Pilot holes and small openings | Cheap and versatile; can wander without a guide |
| Carbide-grit hole saw | Large round openings | Cleaner, controlled cut when used gently; slower than an aggressive toothed saw |
| Step bit | Some larger holes in thin material | Convenient size range; can enlarge a hole too far quickly |
| Coarse-tooth hole saw | Generally avoid for exposed tub surfaces | Teeth can snag, chip the gelcoat, or crack the laminate |
Check the location before drilling
Identify what is behind the tub wall. A hole that reaches a pipe, wire, stud, or waterproofing layer can turn a simple fitting job into a costly repair. Use access from the other side when available. A stud finder may help locate framing, but it cannot reliably confirm that a cavity is clear of plumbing. Follow the fixture’s installation instructions and check that the planned hole leaves enough material around its edge.
Fiberglass dust can irritate skin, eyes, and lungs. Wear safety glasses, a well-fitting dust mask or respirator suitable for fine dust, and gloves. Ventilate the room and keep dust away from drains and fans. Do not use a hammer drill or impact driver for this job. Their hammering or impact action can fracture the laminate and chip the gelcoat.
A steady method for a clean hole
Put painter’s tape over the mark to help reduce surface chipping and make the mark easier to see. Mark the center on the tape. For a small hole, start with a 1/8-inch pilot bit, then enlarge it to the final size if needed. Keep the drill square to the tub wall; an angled hole can make a fitting sit crooked or leave an uneven gap.
Use a cordless drill in its regular drilling mode, with hammer mode off. Select a low speed—roughly 300 to 600 RPM is a useful starting range for a larger hole saw—and keep the trigger controlled. For a small twist bit, moderate speed can work, but avoid running it so fast that the surface heats or the bit smokes. Let the cutting edge do the work. Light, steady pressure is better than leaning on the drill.
With a hole saw, begin at a slight angle only long enough to establish a shallow groove, then bring the drill square once the edge has a track. If the pilot bit slips, a scrap wood guide held firmly against the surface can help. A backing block behind the fiberglass is useful when accessible: it supports the wall as the bit exits and reduces breakout. Do not wedge a block against the tub so tightly that it bows the shell.
Pause periodically to clear dust and let the bit cool. If the bit binds, stop the drill rather than twisting the tool sideways. Side loading can widen the cut or start a crack. Finish by easing off pressure as the bit breaks through. Remove the tape and inspect the rim; smooth only minor roughness, since aggressive sanding can damage the surrounding gelcoat.
Mistakes that damage tubs
The most common failure is drilling too fast with a dull or coarse bit. The surface may chip before the hole is complete, or the heat may discolor or weaken the laminate. Another is using a large bit immediately: it can skate across the curved surface before it bites. A pilot hole, tape, and a controlled start reduce that risk.
Do not assume a clean-looking opening is watertight. The fitting’s gasket or sealant must be installed as specified, and the cut edge should not be left exposed if the fixture instructions call for sealing it. If the tub flexes significantly, the hole is near an edge, or you are unsure what lies behind the wall, stop and get access or professional help before drilling. A modest bit is inexpensive; repairing a cracked tub or a leaking connection is not.