Epoxy fiberglass over polyester filler?

[John_C]

]Hi,

I'm building a 13' fisherman (My first boat) and I'm just about ready to fiberglass, using the appropriate epoxy fiberglass kit from glen-L. I was reading over the instructions on glen-L's "how to fiberglass a plywood boat" page and and noticed that it says "Do NOT use polyester-based putties" for patch work. I just finished filling in every single nook and seam on the hull with Bondo automotive filler. After a quick search I'm pretty sure that Bondo is polyester-based, and now I could just kick myself. Is it possible to use epoxy resin fiberglass over Bondo? I'm guessing not, in which case, is there anything I can apply over the filler that would allow the fiberglass to bond to it?

Here's what the hull looks like now:


If you have any other suggestions I'd really appreciate it.

Thanks,
-John

[galamb]

You will be fine.

Epoxy will bond to quite a number of surfaces "including" polyester resins.

Just rough up the surface so you will get the maximum mechanical bond from your epoxy.

In general the process won't work the other way around - polyester will not adhere well to epoxy, although WEST Systems has done some testing and has a procedure for bonding polyester gel coat to epoxy.

So don't fret it. I too used (bondo) to do much of my fairing on the hull (because I got a good quantity of bondo for nothing) - I went over top of it with fiberglass cloth and epoxy (EAST system, using the dry cloth apply method) - no issues, no lifting cloth, no bubbles, no lack of bond - nada..

[Bill Edmundson]

Several of us have used Bondo under epoxy. I'm not saying that it is the best. I don't like polyester products. They go off too fast. But, I try to limit my exposure to epoxy.

Bill

[Dave Grason]

The problem is that epoxy is not available in just ANY hardware store and Bondo is available EVERYWHERE. And, if you're like me, when you're working on your project, you are in that special mental place where you are getting things done and you just don't want to stop. If you insist on using epoxy, you then have to order it online and it will be several days to a week or so before it arrives. By that time, you've lost your momentum.

[kamisue]

Bondo makes several fillers including the automotive. Check their website and you'll see. They might all be the same just marketed for different purposes for all I know. The stuff does kick off fast. I have several grey hockey pucks to attest to that.

[John_C]

You will be fine.

Epoxy will bond to quite a number of surfaces "including" polyester resins.

Just rough up the surface so you will get the maximum mechanical bond from your epoxy.

In general the process won't work the other way around - polyester will not adhere well to epoxy, although WEST Systems has done some testing and has a procedure for bonding polyester gel coat to epoxy.

So don't fret it. I too used (bondo) to do much of my fairing on the hull (because I got a good quantity of bondo for nothing) - I went over top of it with fiberglass cloth and epoxy (EAST system, using the dry cloth apply method) - no issues, no lifting cloth, no bubbles, no lack of bond - nada..

Thanks! I had a mini-heart attack when I first thought I had messed up. Now I can fiberglass stress-free lol

[jeff k]

As long as the bondo is cured, no worries my friend. Just scuff it with sand paper and your good to go. Epoxy will stick just fine.

[jcallends]

Plain polyester resin without any additives will air cure over a period of time. To counteract this the makers of resin add powered wax to the resin, The wax having a lower gravity than the resin will rise to the surface of the resin and prevent the contact of air and resin. Since the wax is relatively inert it has no effect on the curing of the resin but will also rise to the surface of the cured resin, obviously this will prevent other materials from sticking to it. Any RCers on this blog that have used finishing resin (polyester) on their models will know exactly what I am talking about. I do not know if Bondo uses this type of resin or not but it would be prudent to prep and clean any polyester surfaces in your build.

[Ibrew2be]

I worked in the polyester resin industry as a university student more years ago than I care to count, and well remember the use of wax in resin formulations.

Wax is used as an "air dry" additive in laminating resins, the resins used with glass fiber to go on over top of the gel coat to build up the structure of the hull. Polyester resins cure via a chemical reaction known as a free radical reaction. (The "catalyst" which is added to polyester resin is usually an organic peroxide such as MEKP, methyl ethyl ketone peroxide, which generates free radicals when added to the resin and thus commences the reaction which leads to the resin gelling and curing.)

The oxygen in air stabilizes free radicals which makes them less reacitve and thereby inhibits the gelling and curing of the resin at the air/resin interface, and can leave the surface sticky for an unmanageably long period of time. The wax provides a barrier between the air and the curing resin, allowing the reaction to go to completion, and hence no stickiness. Wax is added to laminating resins only. Resins such as gelcoat, for example, do not use wax, because you want the laminating resin to be able to bond to the gelcoat.

Similarly, the resins manufactured for use in "car patch" (generic term) compounds also do not have wax added. If wax were used, it would interfere with bonding of one layer of car patch compound with the next, in situations where multiple layers are needed.

Polyester resin will harden on its own over time because the cross linking agent, styrene, is in the resin formulation from the beginning. Styrene functions not just as a cross linker, but also as a viscosity reducer, to get the resin into a viscosity range where it can be sprayed or brushed. Polyester resins of all types are formulated with "inhibitors", which function as free radical scavengers, to slow down the inevitable and give the polyester resin an acceptable shelf life.