How to repair worn screw holes in fiberglass?

mitiempo · Jan 30, 2015

Fill the holes with epoxy thickened with colloidal silica, let cure and redrill.

Ken13559 · Jan 31, 2015

mitiempo said:
Fill the holes with epoxy thickened with colloidal silica, let cure and redrill.

Would that be the 2-part epoxy that one knead like dough?
What is colloidal silica?

kloudie1 · Jan 31, 2015

Maine Sail has a great "How-To" here: http://www.marinehowto.com/
scroll down to the one on sealing deck penetrations to prevent rot.. Works fine for loose holes.
Another member had posted a similar method using a piece of heavy wire with a 90 deg bend on the end, chucked in a drill to hog out the core around the hole without enlarging the hole further.. I don't remember who posted it..

svchickadee · Jan 31, 2015

Use bolts instead of screws if possible.

Jackdaw · Jan 31, 2015

svchickadee said:
Use bolts instead of screws if possible.

that.

Allan12210 · Jan 31, 2015

Ken13559 said:
What is colloidal silica?

Also called Micro balloons. Very light weight, micron size silicon dioxide spheres used to thicken and strengthen epoxy.

RichH · Jan 31, 2015

Most 'screws' used on boats are 'wood screws' ... that can easily tear out of fiberglass.

Before you go to the bother of 'filling' the holes ..... go to a hardware store and seek out 'sheet metal' screws that exactly match the 'head size' of your pulled out wood screws.

You'll find that such sheet metal screws have a thicker/larger diameter at the threads, more open spaced threads, threads that run all the way from the tip to 'flush with the head" (many wood screws do not - they have a 'shoulder' or missing section of threads AT the head).
The 'fit' will be tighter, you wont have to drill new holes, and wont be as vulnerable to 'tearing out' of the fiberglass.

wufibugs · Jan 31, 2015

I have never really understood the fascination with wood, or even sheet metal screws in fiberglass. I always wondered why one could not find something like a drywall anchor in stainless or nylon that would expand behind the fiberglass. Wouldn't that provide stronger holding power, use a machine screw, and be removable and reusable? I have seen them in nylon, but not stainless. The nylon ones don't seem to stay tight against the back when the screw is removed like the zinc anchors do, but zinc plated is not suitable for the marine environment.

thinwater · Jan 31, 2015

Allan12210 said:
Also called Micro balloons. Very light weight, micron size silicon dioxide spheres used to thicken and strengthen epoxy.

No.

Micro balloons are what they sound like; hollow, weak spheres that make a light weight filler that is easy for fair.

Colloidal silca (Cabosil M5, West system 406) is chemically similar to very fine sand, but more like carbon black in that it is formed in atom-sized amorphous agglomerations. It makes a very hard, very dense filler that is difficult to sand and very strong.

Micro balloons (West 407) would be a very poor choice for this. West system 404 or 406 (or their equivalents) will work.

thinwater · Jan 31, 2015

Machine screws and self-tapping screws can actually work well, within proper limits. The FRP must be thick enough.

Machine screws, for example, must be at least 1/4", course thread, and threaded through 2x diameter of high density FRP. That done, they will break before pulling out. Unfortunately, they are generally used poorly.

That said, bolts are simpler and dependable.

centerline · Jan 31, 2015

hdhr289 said:
Looking for "how to" advice... The screws holding one of my bimini stainless mount brackets to my cabin top have pulled from the fiberglass. What is the best way to repair so I can reattach the bracket with screws? Should I just fill holes with gel coat then redrill pilot holes and attach again with screws and 4200/5200? Would filling the holes just with 4200/5200 and a couple wooden toothpicks to give the screws something more to bite into be a good idea? Would applying either 4200 or 5200 between the entire surface of base of stainkess bracket and fiberglass cabintop before reinstalling screws be recommended?

Bolts would be the best solution, but if that isnt what you want to do get some epoxy putty or "marine tex", which is an epoxy paste and fill the hole. Its best to use something as a backer, no matter what the material is or how thick or thin it is, as it will allow for a better filling and backing of the hole and a much stronger repair.

As others have said, you can make your own epoxy paste or putty, but unless you need large quantities of it, its more convieniet to buy the stuff and keep in in the tool box. Its just handy stuff to have around.

Rick Templeton · Jan 31, 2015

Any time you are able to access the area behind the fiberglass it is best to add a wood backer plate. I coat the wood with epoxy and attach it the inside with another coat. When it sets you can fill and redrill the hole. With the wood backer the fastener will hold much better. If I can't reach the area behind the fiberglass I use a hollow space anchor called a rivnut. They are a tubular insert that are placed into the the hole with a type of plier tool. Once inserted the pliers are squeezed together and the cylinder is collapsed leaving a ballooned bulb behind the fiberglass and a rim around the outside. The tool is removed leaving a threaded insert behind which will receive a bolt. I always use locktite on the bolts when they are installed. You can pick up a set at any upper scale hardware store for around 80$.

Ken13559 · Jan 31, 2015

I'm trying to secure the wooden covering at V berth under anchor locker. No access to back when cover is mounted. Its either i fill the enlarged fibreglass holes with epoxy or glue a hard wood behind the hole so the wood screw cab bite the backing wood i.s.o. fibreglass.
Can I use JB Weld epoxy to stick the wood to fibreglass? Its a very strong 2 part epoxy.

Terry Cox · Jan 31, 2015

Rubber Anchor

The forward deck grab rails on our P42 were leaking rain water into the forward cabin and a couple of screws were not holding. Removed the rails, cleaned the holes and used a rubber expanding anchor nut similar to the metal ones people use in sheetrock. These anchor nuts are designed for the industrial environment.

I did not want to cut open then close up the headliner to place a backing plate.

I rebedded the rails with 3M-4200 and used those rubber expanding nuts to secure them to the deck. That was eight years ago. Very strong holding and no leaks. See link below.

http://www.lsengineers.co.uk/12-x-75-rubber-nut.html

gettinthere · Jan 31, 2015

West systems 610 epoxy is a nice choice for this application.

If you use the tip, its a one shot deal. I use popcicle sticks to 'slice' off a bit of each part of the epoxy and then mix it up before applying. Has held some wood screws into fiberglass under quite a bit of load.

mitiempo · Jan 31, 2015

gettinthere said:
West systems 610 epoxy is a nice choice for this application.

If you use the tip, its a one shot deal.

You can buy extra tips inexpensively.

gettinthere · Feb 1, 2015

mitiempo said:
You can buy extra tips inexpensively.

But I'm a sailor...popsicle sticks are FREE!

jeanharvey · Feb 2, 2015

hdhr289 said:
Looking for "how to" advice... The screws holding one of my bimini stainless mount brackets to my cabin top have pulled from the fiberglass. What is the best way to repair so I can reattach the bracket with screws? Should I just fill holes with gel coat then redrill pilot holes and attach again with screws and 4200/5200? Would filling the holes just with 4200/5200 and a couple wooden toothpicks to give the screws something more to bite into be a good idea? Would applying either 4200 or 5200 between the entire surface of base of stainkess bracket and fiberglass cabintop before reinstalling screws be recommended?

I would drill a bigger hole. 5/16", then fill the hole with apoxi, dirll a pilot hole.

RADIOFLYER · Feb 2, 2015

Mounts for Biminis and dodgers can have a lot if stress on them (thunderstorm winds). Because of this I would recommend not using shirt cut repair using toothpicks, plastic mollys. They won't be full strength and they will like allow water ingress and bigger longtime issues.

One of the replies mentioned thickening epoxy mix with 'micro balloons'. While micro balloons are routinely used to thicken epoxy... this type of mix makes the epoxy much softer/ weaker/ and definitely not for strength applications. The micro balloon mix is usually used where you will need to do a lot of shape sanding/ fairing and no major stresses expected.

Straight epoxy is very strong but rather brittle by itself and can easily crack under stress. It's the strength medium like fiberglass that gives the epoxy mix it's great overall strength properties. In small repairs you can't fit the usual fiberglass cloth. So fill stripped screw holes with epoxy mix & cut up fiberglass threads from scrap fiberglass cloth. First root out any soft core under the surface decking described in another reply and fill with your mix. Then tape over with painters tape if it's on a angle to keep it from running out. I use fast epoxy which still takes 30-60 minutes to fire off. (Preheating the Part A and Part B and the repair area to slightly warm to the touch will significantly speed up the curing time).