Minimizing bond failure when fiberglassing over original polyester

What can be done to reduce bond failure when fiberglassing over old polyester (under significant loads) when depressions pitting etc remain at limit of grinding?

In other words, how to minimise or avoid epoxy bogging and maximise fiberglass to fiberglass contact.

Here’s an example:

Re-tabbing deck/bulkhead joint of collision bulkhead under shear load (heavy displacement mono).

Spec provided is R10 fillet, 3 layers of EDB600 @±45 epoxy laminating resin.

After grinding down the original polyester tabbing with 36 grit, pockets of depressions, pits etc remain (photos below).

 

To avoid epoxy bogging you need to grind the holes to avoid air pockets . After that use multilayer of low gramature glass (which you probably don't have) and some pressure.
Microbaloon epoxy is a good solution .Why don't you want to use it?

 

Photo 3 requires grinding. Small holes require simple filler. I prefer to add the filler just prior to glassing. If you want to do it before; you’ll meed to make the filler sandable, so not just cabosil.

To avoid cracking of the filler, avoid filling holes much greater than 1/2” ^3. The areas pictured at 1/4” deep should not crack. I’m not sure what you mean by ‘bogging’ as not a term I use, but I believe you mean leaving air in the repair.

I make a sandable filler with about 80/20 balloons or spheres to cabosil. And I mix the dry about 2:1 to epoxy, or a bit more to avoid slump. After mixing, move the material to a hawk and keep it thin to extend working time over the 100gsm ratings. Avoid setting the hawk or work in direct sun. To improve bonding and reduce air entrainment, brush the depressions with clear epoxy 30-60 minutes prior to adding the fillers. At first, it may seem less than ideal as a bit messier, but you will achieve better bonds. Make several passes over the areas and now and then poke deeper depressions with the point of the trowel if it seems a bubble has formed. You can add glass to the mix, but I don’t like to because it makes troweling much more difficult. However, it can add body to the mix for large repairs or if you want some strength.

These holes will need to be ultra dry before you start or blisters may form. If the boat is outside; use a hair dryer on all the areas, or a heat gun and great care to not burn down the marina on a good day well before adding resin.

 

It's not going to make any difference what you do.

People misunderstand this aspect of repairing older boats all the time.

The epoxy and it's bond to the laminate is only as strong as that old existing laminate, exceeding the strength of the old laminate provides no benefit. What happens is the new epoxy laminate and it's bond will not fail, it will just move the failure point into the existing laminate. That's why most repairs are done with similar resin systems as to what the original build was, you don't really gain anything unless you needed some additional strength in that area. And additional strength isn't needed very often, typically a rebuild is required due to rotten wood not a laminate failure, or it was from a collision or very poor workmanship.

In this case sand those low spots and fill or not fill, it makes no difference to the final result. In fact even doing nothing to them will make no difference.

I personally did this type of destructive bond testing in the lab for a customer on some bonding products I supplied to them. We started with lower strength resin systems and graduated to stronger resin systems. Once the failure point moved to the existing substrate the stress to point of failure didn't increase, it leveled off.

 

Can you tell me more about these tests? What kind of test was it exactly?

 

We did instron testing in shear and tensile situations. This was for bonding stringers, bulkheads and liners in 36'+ boats.

When I took over this product I did testing to see exactly what we were making and how it compared to other products. I made samples with different catalysts and at different ratios, plus other resins.

The product had been developed for a specific customer about ten years before and they loved it.

The weaker resins used to make putty would fail within the putty itself, as the physical properties of the resin increased the failure would transfer to the laminate on either side of the putty. Once the failure was mostly in the laminate the stress readings didn't increase, they leveled off and stayed within a similar range. Increasing the strength of the resin system used in the putty didn't make a difference after that point.

For better performance the physical properties of both the resin in the laminate and the bonding putty needed to be increased.

 

Grinding removes and cuts too many fibers. Sandblasting will be more effective and easier. I have used sandblasting on repairs for many years with success.

 

More archeological type of approach .

 

For me the reason is also to save time. I charge by the job, not the hour.

 

Generally speaking, was taught to maintain glass on glass where possible on structural works.
A 6Oz. plain weave or something else? Without space for a standard bevel, how would you stack the layers?

 

Cheers FG. Majority of jobs I used a neat coat of 105 to tack prior to epoxy filler combos. Recently I’ve started using epoxy putty, particular Gurit Spabond. It appears to shrink less on cure and the slow hardeners give around 3hrs working time. When pressure troweling the putty there seems to be less air entrapment than the neat epoxy method, but I could be wrong.

 

Thanks for providing the shear and tensile results of the Instron testing…valuable info. What conditions/situations do you see an exception to the failure mode being transferred to the weaker laminate, or in other words avoid using epoxy filler only? Example that comes to mind is treating large hull blisters: epoxy filling (only) the 12:1 bevel vs. laying up glass.

 

Hell of a time saver...been dealing with old hard as granite gelcoats a lot lately.

 

These old polyesters tend to sheen off quickly after sanding. After cleaning with Acetone a light olive colour is left on the rag - polyester is reacting with the acetone.
Do you guys maintain a specific series of steps to prep old polyester for important structural bonds?

 

The polyester does not react with the acetone, short term contact has virtually no effect, long term leads to some degradation of the finish. You are picking up a little of dust from sanding.

And cleaning the surface with acetone does little or nothing to enhance the bond.