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PostPosted: July 13, 2017, 2:33 pm 
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Okay, here's one for the fiberglass-experienced folks.

In the past, on a few occasions, I've tried to 'glass or glue steel or aluminum to molded fiberglass parts, with a 100% failure rate. I don't know if it's my surface prep, remaining mold release agent, or what, but despite cleaning with a battery of different solvents, sanding, etc., nothing ever sticks.

Now, at this point, I have these spectacular fiberglass front cycle fenders for my Locost, made in the UK. They're made of polyester resin & glass, but I don't know the chemicals, mold release agents used, etc. Because I don't want to drill & bolt through those glorious fenders, my plan is to make up inverted boxes of 20 gauge steel (curved, using my new stretcher/shrinker set) which will bolt to my steel fender stays. The closed faces of the boxes (facing up) would be glued to the underside of the fiberglass fenders using 3M 5200 (the hold strength of this stuff is astounding, and it works even better when exposed to water - Cat***ham uses it, or something like it, to attach their cycle fenders). The shear strength of 5200 is apparently 362 psi on fiberglass (much greater on steel). As the steel-to-fiberglass bonding area will be roughly 42 sq. in., the grip strength would be a little over 15,000 lbs. (7.6 tons). And that's just per fender!

My concern is that if I can't get the undersides of the fenders cleaned sufficiently of mold release, etc., the joints could fail (not the glue, but the interface), and I could lose a fender.

I have on hand various soaps, paint thinner, isopropyl alcohol, acetone, xylene, etc., in addition to various sanding products.

Any suggestions as to a sure way, or series of steps to follow, to ensure the bond as secure as the glue?

:cheers:

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PostPosted: July 13, 2017, 2:57 pm 
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I wouldn't trust glue, but that's just me.

I'd make my attachment pieces and bury them under a couple of strips of glass tape set in resin. I did this for reinforcement on my bugeye bonnet and they were going strong after a few years of daily driving use and 10s of thousands of miles.

Something like this
Attachment:
taped support.jpg
taped support.jpg [ 21.89 KiB | Viewed 170 times ]


The flat blue line is the glass substrate of your fender
the inverted U is the support you want to bolt to

The red lines are three layers of glass tape set in resin capturing the support permanently. I've done this for reinforcing stringers in wood glass and glass composite pieces for ages.

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PostPosted: July 13, 2017, 3:26 pm 
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Okay, I get that - but what do you use to prep the existing fiberglass part so you can bond to it? Even bonding polyester resin & glass to existing, molded fiberglass parts (i.e., bonding "like" to "like") has always failed for me.

I don't know if it was a difference in the brand of resin, or what, but we've all seen examples of parts fiberglassed to other parts which simply separated after a while. Clearly, it wasn't a failure on either of the 'glass parts, but rather the bond itself that failed - probably due to incomplete bonding surface preparation.

Just as you suggested, I had originally planned on fiberglassing some aluminum ribs or similar to the underside of the fenders, so they could be bolted to the fender stays, but with my dismal record on trying to do exactly that, I figured the chances of it failing (and my fenders flying off at speed) were approximately 100%.

I did do a lot of research on the 3M 5200 before even considering it. It's a "marine structural adhesive", used commercially in boat building to attach structural parts like keels on ocean-going sailboats, through-hulls, outdrive (outboard motor) boxes to power boat transoms, etc., even without any other means of attachment.

The stuff retains a little bit of flexibility & is considered impervious to vibration, water, solvents, oils, etc. which I believe would make it well-suited for fender attachment. The down side is...don't even think about ever trying to remove anything bonded with it. Ain't never, ever gonna happen. You can use a saw to remove the joint area but, once cured, the glue itself is completely inert & impervious to solvents.

I watched a boat repair show where the presenters doubted the strength of 5200, and decided to test 3M's claims. They had a new fiberglass hull (about a 20' boat), and attached one of those outboard motor boxes (for twin, huge outboards), using only 5200 without bolts, screws, etc. They secured the hull to the concrete with chains & straps, and used a huge fork lift to try to pull the box off.

Eventually, the entire stern of the boat tore off, transom and all - and the 5200 joint remained in perfect condition.

Nonetheless, if the surface isn't prepped correctly, whatever the 5200 is trying to bond to can fail (like residual mold release agents, etc.), and that's what I'm worried about.

I know some of you experienced folks have bonded fiberglass or steel parts to existing fiberglass parts with success...but how did you prep the surface to ensure a good bond?

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PostPosted: July 13, 2017, 3:45 pm 
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In my experience the molded parts I've purchased were polyester resin unless it was a carbon fiber part. Those were always epoxy.

For prep I've had most success with a 2 step wipe down and grinding for a mechanical bonds
Wipe with acetone, let dry
wipe with denatured alcohol, let dry
Those usually remove the wax from the surface. Even if a part hasn't seen mold release, the resin itself has wax built in that floats to the surface during curing.

Once cleaned I rough up the surface with 80 grit paper on a DA sander.

Way back when I was building TETANUS, every glass panel was cracked, chipped, and generally beaten up from being dragged all over creation to kit car shows back in the 80's. I had to repair all this panels and the process wat much the same.
clean, clean, grind out the cracks, glass in the repair with multiple layers.

Sorry, I wish I had some pics of the bugeye bonnet


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photo3.JPG [ 77.82 KiB | Viewed 162 times ]
photo2.JPG
photo2.JPG [ 73.92 KiB | Viewed 162 times ]

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PostPosted: July 13, 2017, 3:50 pm 
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That is great info, thanks!!

Your process pretty much confirms what I was planning, but I needed to hear from experienced folks.

Now, I have HOPE!!

Cheers, gentlemen! Have one on me (if you were nearby, I'd be buying you a pint)! :cheers:

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PostPosted: July 13, 2017, 6:05 pm 
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this is what I use for release agent and it washes off with water.
https://www.google.com/search?q=pva+pol ... e&ie=UTF-8

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PostPosted: July 14, 2017, 12:01 am 
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Some good advice here. Many Polyester resins have wax that needs to be removed. I use Mineral Spirits followed with Acetone. then some aggressive sanding disks and some more chemical cleanup.

Also in a production mold, PVS is sometimes used so a good cleaning with water is called for. And glassing or bonding to Gel coat is a no-no. Grind it down to the glass substructure if you are looking for a structural bond to the mold surface side.

As for glueing to fiberglass, instead of fiberglassing, I see nothing wrong with a good structural adhesive after using one or more of the above cleaning methods. I have had good success using Liquid Nails or one of the generic polyurethane adhesives found at the local box stores. Once you have properly prepped the fiberglass surface, then it matters not if you use fiberglass or some other adhesive, so long as it is within the capabilities of the adhesive itself.

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PostPosted: July 14, 2017, 12:54 am 
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Will there be release agent on the underside of your fenders? I would expect release agent on the top, the smooth side that was in the moulds but not on the bottoms which should be the side the manufacturer was laminating from.

3M 5200 is a good adhesive but it is a little kinky to work with. I used it during the restoration of my current sailboat for bonding new, replacement wood reinforcements onto the original fiberglas as well as some other applications.

The things that I wasn't expecting from 3M 5200 are;
1) it is very liquid as it comes out of the tube and will run out of any joints where there is opportunity before it sets up. In some locations in the hull where the joint could not be laid flat, I would assemble everything then quickly tape up the edges of the joint with painter's tape to keep the 5200 where it was supposed to be. That meant that the tape became very hard to remove afterwards but at least I had strong joints.
2) 5200 takes a long time to "stick" (reach the point where it has a little strength and will actually hold stuff together) and even longer to fully cure. Working indoors in temperatures that were in the high 20'sC during the day and falling to about 18C in the garage at night, "stick" took about 36 hours.
3) because of the two issues above, I found that each joint needed to be solidly clamped together for enough time to allow the 5200 to cure. In the case of my boat almost every joint had bolts going through the new wood pieces so clamping was easy but I had to be inventive with pieces that had no through bolts.

I had thought that the 3M 5200 was going to look, feel and work just like a super-duty silicone seal (a gel) but it's not like that.

I found that the 5200 is tough as nails once it is cured and I did get strong bonding with the fiberglas hull. To prep the hull I sanded it with rough grit paper, then vacuumed it, then cleaned the area to be bonded with acetone (I'm not sure acetone is best for this job but it was on hand and it seemed to work well for me) then stuck everything together. I don't know what the hull is made out of...it's an old Vandestadt & McGruer 17' swing keel sailboat.

Good luck with the stuff. Post pictures!!

Bill

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PostPosted: July 14, 2017, 2:54 am 
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Thanks gentlemen, for all this valuable info! Now I feel I have a plan that will work!

I figure on starting with a soap & water scrub (liquid dish soap), then acetone wipe down, alcohol wipe down, sanding, and a final acetone wipe down. Hopefully, that will prep the surface adequately.

BHRmotorsport - I really appreciate your insights from personally having used 5200. The directions don't explain the odd characteristics of the material, so I'm now much better informed as to how to use the stuff. My original plan was to mount the inverted, curved "boxes", spread the glue on top, then set the fenders onto the glue to cure. Now, I suspect I might be better off to invert the fenders, place the glue, and set the boxes down into the fenders. The glue can't run away that way, and with tape I can make sure the boxes don't shift during curing time. The length of time for cure isn't a big deal - if it takes a couple of days, I'm fine with that, as the car won't be going anywhere for months yet.

Originally, I was tinking of making a couple of dozen holes in the steel boxes, so the glue would ooze through a bit, forming "rivets". I've done this with epoxy before, but it sounds like the 5200 is way too runny for that to work. Oh well, it's a lot less work to omit the holes...and I suspect the strength would be better with an unbroken glued surface, rather than one with a bunch of small breaks in it (i.e., the holes).

Apparently, there are several variants of 5200. The stuff I got is called "fast cure, non-sagging formula, and purports to be thick enough to be worked with tools "to the desired appearance". In my experience, you never know until you puncture the end of the tube, though.

Thanks to your insights, I have a much better idea what to expect, and what to plan for!

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PostPosted: July 14, 2017, 11:28 am 
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Zetec7 - I was using the stuff pictured below, sourced from Canadian Tire. I'd try the "5200 rivet" method you describe, particularly if the version of 5200 that you're going to use is toolable, that sounds like it should work well.
Attachment:
Screen Shot 2017-07-14 at 10.26.31 AM.png
Screen Shot 2017-07-14 at 10.26.31 AM.png [ 72.12 KiB | Viewed 97 times ]

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PostPosted: July 14, 2017, 11:33 am 
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Thanks! Yeah, there are, apparently, several formulations of 5200.

I'll try the stuff I have, and will update this thread once I find out how well it works.

The is a GREAT forum!! :cheers:

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