how to repair screw holes in fg for re-use

[bony]

Hi everyone,
I got a row of s/steel screw-in snap in press studs going around the gunwhale of my boat for securing the cockpit cover. Some of the screws have pulled out of the fibreglass and will no longer hold when screwed back in. I need to repair the holes and re-use the screws, but as it is a fully enclosed gunwhale I cannot get to the other side of the f/glass to do a good solid repair by reinforcing the area. I can't use bigger screws, and I've tried to fill the holes up with various glues and sealers but they're not very succesful.



So, I need some advice on a better way to fix the holes that will allow me to replace the screws, without having them pull out again. Any ideas?




ps. The f/glass is only between 3/16" to 1/4" thick for most of the screws.

[pcmm]

epoxy with a bit of cabosil in it to thicken it. use a syringe to apply to minimize the mess afterwards. Once hard, sand to a bit lower than flush and then brush on a bit of gelcoat to cover

[Kalimniosjohn]

Since dealing with epoxy is probably necessary, might as well do it right and drill the holes oversized and fill with thickened epoxy. Then once cured re drill the holes to the proper size. This way you won’t ever have issues with water intrusion.

[US1Fountain]

That’s exactly how I repair several stripped snap screws this spring. Thickened epoxy and redrilled with the correct size bit

[Shrew]

I've had good results from this stuff:

https://www.defender.com/product3.js...992&id=2841319

It's a two-part epoxy that is needed together like a clay. No thickener needed.

[bony]

I've tried varies glues and putties similar to one Shrew suggests but they're not really successful because in my case they need to have pressure behind them inside the gunwhale to help them stick to the inside of the g/w wall. With no other access than the screw hole itself it's doesn't work out.



I guess drilling oversize and doing it properly with thickened epoxy is the way to go after all.



Thanks for the replies everyone.

[basssears]

Practical Sailor just did some tests on this, turns out (according to their tests) that thickened epoxy is nowhere near as strong as straight epoxy... I guess the thickeners (they tested sawdust, micro ballons, and colloidal silica) create void spaces that decrease the strength of the epoxy.

Their recommendation is straight epoxy, or if you really need extra strength epoxy with cloth pushed into it.

Thickened epoxy pulled out at 55-90 lbs of pull (depending on what it was thickened with), epoxy only was 400 lbs and epoxy + fiberglass cloth was 800 lbs.

Full article is here:
https://www.practical-sailor.com/iss...r_12405-1.html
... but you need subscription to access it.

-- Bass

[guyrj33]

Originally Posted by basssears

Practical Sailor just did some tests on this, turns out (according to their tests) that thickened epoxy is nowhere near as strong as straight epoxy... I guess the thickeners (they tested sawdust, micro ballons, and colloidal silica) create void spaces that decrease the strength of the epoxy.

Their recommendation is straight epoxy, or if you really need extra strength epoxy with cloth pushed into it.

Thickened epoxy pulled out at 55-90 lbs of pull (depending on what it was thickened with), epoxy only was 400 lbs and epoxy + fiberglass cloth was 800 lbs.

Full article is here:
https://www.practical-sailor.com/iss...r_12405-1.html
... but you need subscription to access it.

-- Bass

I wouldn't use any of those additives to make a high strength epoxy filler, I suggest West High Strength Filler. I think it's ground limestone or something similar.
Sawdust, microballons, colloidal silica none of these are recommended for high strength epoxy fillers. What were they thinking?

[basssears]

Originally Posted by guyrj33

I wouldn't use any of those additives to make a high strength epoxy filler, I suggest West High Strength Filler. I think it's ground limestone or something similar.
Sawdust, microballons, colloidal silica none of these are recommended for high strength epoxy fillers. What were they thinking?

Reading the whole article I don't think they were expecting those to work, I think they were trying to make it clear that they DON'T work.


-- Bass

[Cheechako]

It shouldnt take much really. I would never drill big hole to repair a small hole. You can repair the holes and they will be covered back up by the fitting. Epoxy filler is terrible for drilling into and has little strength, breaks out readily etc. If the screw still fits but wont tighten, just plain Iso 5 second glue should do it if you clean out the hole.
They make adhesive in a caulking tube now days that is extremely strong. I forget what it is called but have even been used in lieu of welding metals. I have tested it on aluminum boat hulls with stringers that were normally welded to the hull. We could not break it off with a sledgehammer. Frankly though, I'd just use JB Weld, Liquid Steel etc. "Kitty Hair" might work fine and have good filler.
Another method is go buy brass "threadserts". You drill a slightly larger hole and thread them into the hole. Then buy fittings with machine thread screws in lieu of wood screw threads. But you really dont need to go this far.

[wrwakefield]

Bony,

I always assume it will just happen again, so instead of repairing the hole just to make another with a 'woodscrew', I either install a stainless steel threaded insert [epoxy in place; most expensive and time consuming; need thicker substrate], or a stainless rivet nut [no epoxy needed; cheap and quick.] Both are very effective and much stronger than the original 'woodscrew'.

Choice of above options depends upon the bearing thickness of the fiberglass.

If you only need to do this for canvas snap enclosures, you might also consider the glue-on snaps Sailrite offers. Just glue them over the existing holes and call it a day...

Best wishes resolving this issue.

Cheers! Bill

[guyrj33]

Originally Posted by basssears

Reading the whole article I don't think they were expecting those to work, I think they were trying to make it clear that they DON'T work.


-- Bass

I only had the original post to comment on, don't have access to read the full article.
Perhaps you could share some of the recommendations?

[basssears]

Originally Posted by guyrj33

I only had the original post to comment on, don't have access to read the full article.
Perhaps you could share some of the recommendations?

Didn't mean my original comment to sound weird, I just don't think they were expecting the fillers to work but they know so many people do it that they felt they should address it.


All of their results depend on properly drilling pilot hole BTW.



Not too much more to the article than their final recommendations. Basically they found that any filler created voids that weakened the epoxy around the threads. Anything with filler in it stripped out under load (55# load for epoxy + sawdust, 75# load for epoxy + microballons, 90# load for colloidal silica). Straight epoxy and epoxy with fiberglass cloth pushed into it never stripped, the whole plug pulled out of the skin. Epoxy only failed at 400# load, epoxy with fiberglass cloth failed at 850#.


For the epoxy plus cloth they recommend starting with neat epoxy (saves concerns about wetting everything out) then stuff a strip of cloth into the hole with a small screwdriver / skewer etc... most of the air bubbles will be displaced by this stuffing but you might still have to agitate a bit to get the rest of the bubbles out. Let it cure with the cloth standing proud then sand.


Of course, they do point out, these are the best techniques they found assuming you're sticking with using screws, through bolting is still obviously much stronger.


-- Bass

[ZULU40]

drill out hole to a 45 degree cone shape
fill with thickened epoxy

re drill to correct size in correct location

[guyrj33]

Originally Posted by bony

I've tried varies glues and putties similar to one Shrew suggests but they're not really successful because in my case they need to have pressure behind them inside the gunwhale to help them stick to the inside of the g/w wall. With no other access than the screw hole itself it's doesn't work out.



I guess drilling oversize and doing it properly with thickened epoxy is the way to go after all.



Thanks for the replies everyone.

were the original fittings sealed with silicone?
Is that preventing the epoxy from sticking?