Drilling Holes in Fiberglass

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This is one topic in which there seems to be a lot of differing opinion. In this discussion, I will show you how I drill holes in fiberglass. This may or may not be how manufacturers or other boaters do it, but it works for me.

There are two methods that can be employed, the Bedding Method and the Plug Method. While the Bedding Method may be used for holes above the waterline, the Plug Method is superior, and should always be used if the fiberglass laminate is cored, or the hole is below the waterline. Also, when using the Plug Method, it should always be combined with the Bedding Method. In othe words, you should always use Bedding Material for any hole you make, whether or not you create a Plug.

Know your Fiberglass. Before attempting to drill that first hole, you should know that fiberglass is a composite construction. Typically, the fiberglass "sandwich" consists of the following layers:

The sandwich described above is simply one example of the almost countless formulations boat builders use in modern fiberglass boats. The important point here is to know that any water that gets into the hole you drill can wick into the coring material. This water intrusion can cause severe damage to the hull and deck.

Crush resistance. One issue to be aware of is the crushability of the coring material, especially with thru-bolting. Depending on the laminate, it is possible to install a fitting, then tighten it down so much that the fiberglass sandwich becomes crushed. This may occur immediately, months, or years after the installation of the fitting. This is so much of an issue that some manufacturers do not core around certain areas of the hull where fittings, such as cleats, will be located. They either leave the areas 100% fiberglass, or use plywood for a backing.

As a boat owner, perhaps you want to install a VHF antenna. Chances are - especially if the boat has an electronics package option, that the boat manufacturer may have created an area in the laminate that has a mounting surface for the antenna mount. Perhaps the manufacturer put a plywood backing in an area intended for the antenna, or at the very least, left out any coring so that the laminate is not crushed (not to mention protected from water damage). If such an area is not found and you need to drill into a cored area, you must provide some crush resistance to any thru-bolted fittings you will install. In some cases, this may be satisfied by simply using a backing plate, but often, especially in a high-load area such as a cleat, you should also remove any coring that exists around the bolts, which can be satisfied by using the Plug Method.


Bedding Method. In this popular method, you simply drill a hole, apply bedding compound to the screw, bolt, or fitting, then insert the item in the hole. You must apply bedding compound to the threads, in the hole, and under the head of the fitting.

Use a liberal amount of bedding compound under the hole to ensure water cannot invade the hole. The following compounds are commercially available:

Note that the primary difference between 5200 and 4200 is the strength of the adhesive, with 4200 having about half the strength of 5200. The 5200 adhesive will result in a permanent bond. The 4200 or 101 is weaker, and will allow easier removal of the fitting.

Both 5200 and 4200 are Polyurethane sealants. Consult the 3M documentation for which one fits your application.

Some experts only recommend 5200 sealant - especially when bedding cored laminate.




Plug Method. This is highly recommended for any hole below the waterline, or in any hole that has coring material, especially if you are thru bolting and need to provide some crush resistance to the laminate. Refering to the drawing below:

Step 1. Drill the hole a bit larger than you need. A good estimate is a minimum of 50% larger. For instance, if you need a 1/2in hole, drill a 3/4in hole. If you are going to screw a something into the hole, a larger hole is even better.

Step 2. Create a plug by sealing one end of the hole, then pour mixed epoxy into the hole - thereby creating a plug. There are many formulations of epoxy, and generally, the slower curing epoxies are stronger as they have more time to soak into the laminate.

Step 3. After a minimum of 24 hours (longer if a slow cure epoxy is used), drill the hole in the center of the plug the finished size you need. This creates an epoxy sleeve and completely protects the raw edge of the fiberglass sandwich.

Step 4. Attach the screw, bolt, or fitting with bedding compound by using the Bedding Method described earlier.

With cored laminates, An even better method is to bore a hole several times the diameter of the finished hole, which will remove an extra amount of coring material. For instance, for a 1/4 in finished sized hole, make the counterbore 3/4 inches in diameter or larger.

This modification is superior for two reasons. First, it gets the wick-prone core material farther away from the hole, and secondly, it provides an increased surface area of non-crushable material under the bolts.

Drilling the hole. Gelcoat is a thin, brittle coating, not unlike an eggshell. If you simply attempt to drill a hole into the fiberglass, you will have extreme difficulty in drilling holes without cracking the Gelcoat. The method I use, which works about 99.9% of the time is to put the drill in reverse, drill through the Gelcoat, then put the drill in forward and drill through the rest of the laminate. The Gelcoat is only about 1/16" thick, so drilling in reverse won't take long. I am not quite sure why this works so well, but probably has to do with taking less "bite" out of the material.

Another consideration to keep from cracking the Gelcoat is if you are going to screw into the fiberglass, then use a countersink or larger drill bit to counterbore the Gelcoat (but not the underlying fiberglass) to a larger diamater (again in reverse) to keep the screw threads from coming into contact with the Gelcoat (as they will surely crack the Gelcoat).

Some boaters like to cover the drill area with Blue Painter's Tape before drilling, but I have not found this to be effective in preventing cracking of the Gelcoat. If you have never tried the reverse drilling technique, you will be surprised in how well it works. However, Blue Tape is great for marking off the holes as well as preventing epoxy from getting on the Gelcoat. Epoxy will stain the gelcoat, and although it can usually be removed with some Acetone, its far better to avoid getting epoxy on the Gelcoat to begin with.


While raw epoxy may be used to fill the holes, better results will be obtained if you use a filler, such as West System's 403, 404, or 405 fillers, mixed with the epoxy to a consistancy of peanut butter. This will also aid in the filler staying put on vertical surfaces. Another recommendation is that if you are screwing into the epoxy plug, the epoxy itself is a bit weak. In this situation, consider stuffing a bit of fiberglass mat into the hole along with the filler (or consider using Evercoat's Kitty Hair), as this will provide more "bite" for the screw threads.

Alternate Methods. A product called CPES (Clear Penetrating Epoxy Sealer) is available from a company called Rot Doctor (www.rotdoctor.com). West Marine also sells a version of this product. This epoxy has a thin consistancy compared to standard epoxy, which allows it to penetrate into coring several inches from the hole. The idea here is to drill the hole, apply the CPES, which will penetrate the coring - thereby sealing it - then use the standard bedding method. The only disadvantage to this method is the time required for the CPES to cure - which can take a few days. But if you have the time, it certainly cannot hurt.

Why all the fuss?

As indicated above, the problem occurs when you have a cored hull. If water gets under the fastener, aided by hydraulic pressure, can wick into the coring material in an area far greater than the hole. The water can remain there for a long time, and eventually rot the coring material. This can result in delamination of the fiberglass, which results in a weakened hull or deck. A "drill-happy" boat owner that does not take the time to properly install and seal any holes can severely damage a boat. The simple addition of snap-fasteners around the hull for a cover can destroy a boat if they allow the coring to become wet.

Final Thoughts

Given the expense and damage water intrusion can cause, especially in a cored hull, properly sealing holes is time well spent. Unfortunately with many of today's production boats, this is an area that should be given more attention. The best boatbuilders will not core sections of the hull that are intended to have a fitting placed on. However, the boatowner does not know where these areas are, and indescriminate drilling for the various doo-dads that get put on a boat can cause serious damage.

Who knew there was so much effort required to drill a hole.


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