Using Bondo for Wood Repair

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charliec

charliec

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Maybe this should go in the "Way off Topic" thread, but here goes anyway. I'm thinking about trying to use bondo to repair a partially rotted joist in the patio cover. If I have to replace the joist, I'm looking at major rebuild. If I can use bondo, I am hoping to be able to build a form out of plywood (or some other material) and fill it with bondo. However, I'm afraid that the bondo is going to bond to the plywood form; then I'm SOL as far as removing the form and shaping/finishing the bondo. Has anyone tried this, or is there a better material to use for a form that won't stick to the bondo?
 
charliec said:
Maybe this should go in the "Way off Topic" thread, but here goes anyway. I'm thinking about trying to use bondo to repair a partially rotted joist in the patio cover. If I have to replace the joist, I'm looking at major rebuild. If I can use bondo, I am hoping to be able to build a form out of plywood (or some other material) and fill it with bondo. However, I'm afraid that the bondo is going to bond to the plywood form; then I'm SOL as far as removing the form and shaping/finishing the bondo. Has anyone tried this, or is there a better material to use for a form that won't stick to the bondo?
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Take the plywood form and cover it in plastic wrap. Bondo shouldn't stick to it.
Just my thinking anyway.
 
The thing to remember here is that wood expands and contracts, bondo does not, so overtime the repair will reveal itself.
 
Is there a special formulation or some kind of additive that I can add that will make bondo more flowable without upsetting the setting action?
 
Make the wood wet. Bondo will not stick. That said, getting your joist dry enough will be a challenge. Plastic wrap might work but make sure the heat from curing isn't too high.

We use Bondo over top of the screw heads when we build fiberglass floors for our freezers, so the fiberglass has something to stick to.
 
Here is how you do it:

Clean the rot out, and let it 100% dry > might have to use a heat gun.

Then brush on fiberglass resin with the hardener to give the filler something to stick to.

Then instead of using bondo use USC Duroglas fiberglass waterproof filler > much stronger.

use wax paper to build whatever forms you need to assist in getting the needed shapes. Wax paper can be adhered to your forms with automotive upholstery Aerosol Adhesive.

When the Duroglas starts to set up firm, pull the forms and wax paper to reveal your finished product .

Can sand, prime and paint repair as needed. Or if you want it natural, just brush on another coat of fiberglass resin and hardener to seal it from the elements. They make boats out of that stuff.

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I've used Bondo to repair wood where the rotted area was just aesthetic. It is no good at all for any structural repair. Very similar to your situation I have chipped rotted areas back to solid wood and used plywood as forms. Waxed paper around the plywood will keep the Bondo from sticking. George Jets has this topic covered well.
 
charliec said:
Maybe this should go in the "Way off Topic" thread, but here goes anyway. I'm thinking about trying to use bondo to repair a partially rotted joist in the patio cover. If I have to replace the joist, I'm looking at major rebuild. If I can use bondo, I am hoping to be able to build a form out of plywood (or some other material) and fill it with bondo. However, I'm afraid that the bondo is going to bond to the plywood form; then I'm SOL as far as removing the form and shaping/finishing the bondo. Has anyone tried this, or is there a better material to use for a form that won't stick to the bondo?
Click to expand...
While it is a hack idea instead of a legitimate fix, can you just add another joist alongside the rotten one? This will get around the rotten one weakening enough to collapse. I've had inspectors advise what we called a "sister joist/stud/rafter" that was attached to a damaged unit and nailed together.
 
Echoing others: Bondo is fine for appearance/aesthetics/non-structural use. An exposed rafter is structural. How much depends on how much overhang there is and spacing. If you need a form to hold enough bondo in place to cure, then it should be repaired with a same size "sister". Here is the basic method:
 
Normal procedure would be to cut the bad part out, and just scab a board onto the side. Option b would be to just scab a board onto the side, and leave the rotten section in. Either one would be more effective than just bondo.
 
charliec said:
Is there a special formulation or some kind of additive that I can add that will make bondo more flowable without upsetting the setting action?
Click to expand...
Plastik honey by evercoat
Or fiberglass resin bondo brand should ride

Sometimes reducer and sometimes denatured alcohol. The latter seem to shrink a little more

Evercoat also makes a glazing coat product that I mix with thicker mud too… green label
 
Do not use Bondo to repair rot in wood. The rot WILL come back. You might as well fix it properly now.
 
I decided to cut out the rotten wood and replace it. It's a better overall fix, and it's not really any more work than trying to work bondo.

For the sake of the discussion though, when I was exploring ways to form for the bondo, I used some scrap 1/8 plywood and wiped a smear of petroleum jelly across it. I let it sit long enough for it to absorb into the wood and then I wiped the excess. I applied some bondo across the board. After it cured, the part that was over the Vaseline part peeled off smoothly. The rest of it bonded tightly to the plywood.
 
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