Fastener Technique

[We're Here]

Looking for advice as to how to make reliably watertight fastener penetrations into and through FRP laminate. I'm hoping for something that works in a variety of situations like: through-bolting an outboard to a glass-plywood-glass transom; a blind fastener like a self-taping screw that goes through the outer laminate and buries the tip into core without penetrating the laminate on the other side; through-bolting a seacock in place; or through-bolting a lifeline stanchion base. I've gouged out core and replaced it will fillers when installing a through-hull but this isn't always a practical solution for the examples that I've cited. I'm not comfortable slathering threads with 3M something and calling it a day either. I'd appreciate any volunteered thoughts on the subject.

[rasorinc]

For bolts drill out one size larger then put a piece of tape over the down side and fill with epoxy adhesive and let harden. When hard drill it out to the correct size insert bolt and WA LA..........I would not wet set bolts or screws an you may never get them out again.

[PAR]

Use a fairly fast cure epoxy (if you're in a rush) and bond the hole as Stan (Rasorinc) suggests. There's a very good reason(s) for this treatment, as I'm sure you know and though a few things have been tried, like placing cotton string under the washer and/or bolt head, magic goo in a tube that peels off in a few years, etc., we've just not found a better method that offers as much as epoxy bonded fasteners (through or other wise). Most fast cure epoxies are fairly weak in comparison to standard formulations.

[We're Here]

Thanks for the advice. I'm assuming that there is a certain thixotropic quality needed. Can you recommend a preferred epoxy (or an epoxy additive [microballoons, etc.]) for this process? Also, is the one drill bit size sufficient given the possibility that the second pass is unlikely to precisely follow the path of the first?

[Saildude]

I tried the "one size larger" drill thing and found out that I usually drill out into the base core so I now take part of the core out at least 1/8 inch before putting in any plug material.

I have seen many recommendations by professionals to go up to 3/4 inch all around larger. One of the reasons mentioned was that depending on the type of fasteners being used the core can be crushed which will then have the plug material fail and allow a water path to the core. This failure would be dependent on the load and type of core.

After having some bad experiences I now error on the side of making the plug of solid material in the core larger and stronger.

And yes the sealant on the fastener will fail sooner or later so I am on a seal all the core penetrations rampage on my boat so the next time the bedding material fails all I will do is get a bit of drip into the interior and not the core.

Read the article here on Core Penetrations and Closeouts for more background.

http://www.bpspecialprojects.com/PDF...f%20files.html

[PAR]

30% to 40% larger in diameter is the usual recommendation for bonding fasteners.

Micro balloons, spheres and other light weight fillers are for fairing only and have no place in a structural application, unless in a very small percentage to ease smoothing a bit. Cab-o-sil, wood flour, cotton flock, silica, milled 'glass fibers (not chopped) and several other assorted materials are employed, depending on application and materials to be bonded. In a typical transom bolt application, my mixture would be thicken to runny ketchup with milled fibers, then add wood flour until it's a very heavy cream, then use silica to stiffen the mixture (peanut butter consistency) so it can't sag out of the hole. I generally don't attempt to force a drill bit through until it's well past tacky and often I hand twist it through so I don't disturb the weak bond that's still curing. Sometimes it's best to let it get fairly stiff, but still rubbery, before you attempt to drill.

[Stumble]

My personal preference is a little more time consuming than above, but basically the same idea. I drill pilot holes from the 'finished side' usually the outside, then from the back side of the mounting use a paddle bit to drill out the core. Which size to use is debatable, but I always go for 1/2 inch larger than the bolt that is going to be applied. This method is more time consuming, but it has the advantage of leaving more of the original glass in place, and the plug gets mechanical support from the shelf it rests in. It also leaves smaller holes to disguise on the finished side.

I typically do all the drilling and filling on one day, then come back and drill the last hole and actual mounting the next day. This way the epoxy has a full day to cure. I don't know it is necessary, but it makes me sleep easier knowing that I didn't mount something to soon.

Since this work is rarely cosmetic in nature, and the material cost is negligible I just use silica structural filler and epoxy. I am sure Par is right that milled fiber would be better, but that stuff always winds up in my skin and irritates it for days. And I don't believe that the structural loss is significant enough in this application to worry about.

[PAR]

Silica alone tends to be brittle used in this fashion. If milled fibers bother you (it's the best glue on 'glassed surfaces) then use cotton flock (West 403) and some wood flour for a good bond to both the transom skins and plywood core.

[Charly]

Originally Posted by PAR

Silica alone tends to be brittle used in this fashion. If milled fibers bother you (it's the best glue on 'glassed surfaces) then use cotton flock (West 403) and some wood flour for a good bond to both the transom skins and plywood core.

Hey PAR, how is cotton flock different from regular cotton (Ive never seen any), could you make your own from something out of your medicine cabinet? and why is cotton better than chopped glass fibers for this application?

[We're Here]

After the thoughtful advice provided by forum members, I asked Gougeon Brothers (West System) for their opinion and got this:

I would recommend after you drill the oversize holes to coat the inside of the holes with just unthickened epoxy for the first coat. While still tacky fill the holes with 105 system and 406 Colloidal Silica or 404 High-Density Filler to a non sag consistency like peanut butter, and then drill holes to the desired size after fully cure.

[PAR]

West 404 is purely calcium metacilicate which is also quite good in bonding fasteners and West System's preferred reinforcement agent for bonding fasteners.

I recommended cotton flock (West 403) instead of milled fibers, because of Stumble's dislike of milled fibers. If you want to avoid getting these little bastards in your pores (the itching thing) then use hair spray on your exposed skin, particularly areas that bend like wrists, elbows, around the neck, the waste band, etc. This will seal the pores and the fibers can't get in. Of course wash off the fibers as soon as possible, with cold water. Another trick is baby powder, used in the same locations and it does the same thing, though not quite as effectively as hair spray.

[Stumble]

Par I will have to look into the 403. I am so sensitive to glass I have to wear painters overalls, two pairs of gloves, and tape myself in before I can get to grinding. Ihave no idea why, but glass fibers stick with me for days no matter what I do.

[PAR]

You're just a woosie Stumble . . . Try the hair spray trick, it works very well and you'll smell nice afterward too.

[Stumble]

LOL. I know it sucks... last time I did glass work I had a skin rash that lasted a week.

[PAR]

Swave Extra Hold hair spray dude . . . it's cheap and I have some on my shef incase I'm in need of a primp.