Will Fiberglass Resin Stick to Bondo?

[Dockhead]

I'm repairing a hole in a little rowboat I use to get to my mooring. I've never worked with fiberglass before. The area around the hole is filled with Bondo (polyester auto body filler). Is it essential to get out every speck of it? It's very tedious.

[Sailmonkey]

Polyester resin will adhere nicely with scuffed up bondo........at least as hard as I've ever tried to pull it apart. Epoxy I have no idea about, but don't think it should be a problem.
Your bigger problem will be how hard the bondo is adhered to what's below it. Bit being a soft fairing compound will come off easily with the application of a sanding disc on a 4" grinder.

[Pete7]

Make the mix "hot" because the air temp is 7.c at the moment otherwise it may not go off. Use a wide shallow dish to mix the resin which helps slow the reaction. If it goes off half way through the job, just ditch it, if you try to use it as it goes plastic then it will end in tears.

Shout if you need a dremel or angle grinder.

Pete

[Sam Plan B]

If the bondo has cured for more than 24 hours polyester resin will not chemically bond to it. However, if as has been said, you rough up the cured bondo with sand paper, you can achieve a mechanical bond with polyester or with epoxy resin.

[Dockhead]

Originally Posted by Pete7

Make the mix "hot" because the air temp is 7.c at the moment otherwise it may not go off. Use a wide shallow dish to mix the resin which helps slow the reaction. If it goes off half way through the job, just ditch it, if you try to use it as it goes plastic then it will end in tears.

Shout if you need a dremel or angle grinder.

Pete

Thanks, everyone! What a wealth of knowledge on here. The job seems to have gone ok. I managed to get most of the bondo off. I did use about 150% of recommended hardener for a hot mix, but it didn't kick while I was working. We'll see if it cures properly.


What a cool material! Amazing how the glass mat transforms after it absorbs the resin. First time I've ever played with fiberglass.

[Pete7]

Unless it is covered with plastic it will cure but have a sticky surface once hard. This can be removed by lightly sanding. Covering in plastic stops this and can be peeled off cleanly afterwards.

Temperature is your problem at the moment, a small patch just doesn't generate much heat.

If it is still accessible then cover it up and add a hot water bottle on top if available.

Pete

[Dockhead]

Originally Posted by Pete7

Unless it is covered with plastic it will cure but have a sticky surface once hard. This can be removed by lightly sanding. Covering in plastic stops this and can be peeled off cleanly afterwards.

Temperature is your problem at the moment, a small patch just doesn't generate much heat.

If it is still accessible then cover it up and add a hot water bottle on top if available.

Pete

I thought it would cure, albeit slowly, as long as at least 5 degrees?

I could go hit it with a heat gun, but don't know if that would help

[Lake-Effect]

DH - you didn't mention what the rowboat was made of.

Depending how much/where the Bondo was, I would have ground it all out, even back to original material. If someone slapped Bondo in there, chances are the prep wasn't so great either.

I've been using epoxy for most repairs ($$$, but has great bond) but that's mainly because I bought a gallon of it 4 years ago and there's still some left. I even patched the rust-holes on my truck with glass cloth and epoxy. Should outlast the truck.

[roverhi]

Heat will help cure the resin. A heat lamp would be my weapon of choice. Holding a heat gun on the site gets tiresome quickly.

[minaret]

Originally Posted by roverhi

Heat will help cure the resin. A heat lamp would be my weapon of choice. Holding a heat gun on the site gets tiresome quickly.

Heat lamps are dangerous. Don't ask how I know. Five minutes with a heat gun is fine.

[Dockhead]

Originally Posted by minaret

Heat lamps are dangerous. Don't ask how I know. Five minutes with a heat gun is fine.

Thanks!

[Dockhead]

Originally Posted by Lake-Effect

DH - you didn't mention what the rowboat was made of.

Depending how much/where the Bondo was, I would have ground it all out, even back to original material. If someone slapped Bondo in there, chances are the prep wasn't so great either.

I've been using epoxy for most repairs ($$$, but has great bond) but that's mainly because I bought a gallon of it 4 years ago and there's still some left. I even patched the rust-holes on my truck with glass cloth and epoxy. Should outlast the truck.

It's heavy-laid fiberglass. An old, squatty, little rowing dinghy. Would sink like a rock if swamped. It has dinghy wheels in the transom which stick far out (big lever arm). One of them pulled the bolt through the transom, leaving a hole.

In the event I got nearly all the bondo out. It was bloody tedious as I did not have anything coarser than 120 grade sandpaper.

Interestingly, the resin which came with the fiberglass repair kit smells exactly like fresh Bondo. I suppose it must be the same material (polyester?). Brought back somewhat unpleasant memories of car body repairs (restoring an old MGA) from my misspent youth.

[Dockhead]

Originally Posted by Dockhead

It's heavy-laid fiberglass. An old, squatty, little rowing dinghy. Would sink like a rock if swamped. It has dinghy wheels in the transom which stick far out (big lever arm). One of them pulled the bolt through the transom, leaving a hole.

In the event I got nearly all the bondo out. It was bloody tedious as I did not have anything coarser than 120 grade sandpaper.

Interestingly, the resin which came with the fiberglass repair kit smells exactly like fresh Bondo. I suppose it must be the same material (polyester?). Brought back somewhat unpleasant memories of car body repairs (restoring an old MGA) from my misspent youth.

Thanks to everyone for all the good advice. The repair - four layers of glass mat - cured hard as a rock! What a marvelous material! Dinghy is back in the water, not leaking a drop, and has already executed its first crew transfer!