Dang! Bondo question....

[Mr. Alligator]

I have been working on one of my first Bondo projects...

I generally used too much hardner, so my mixes did not last long. So the relatively small (one foot by two 1/2 foot) and relative thin (less than 1/4 inch total, at thickest point) project is really twelve or fifteen small batches of mud.

Maybe no problem... but it just occurred to me that, in between batches, I cleaned the area and sanded with 400 grit discs. Instead of something more coarse.

Should I sand it off or heat it and scrape it off, or just keep going? Of course, I do not want it to fail.

Thanks for your help!

 

[shucker1]

Why not just run back over the top lightly with 220 to rough it up a little? You need something for the next coat to stick to.

The trick is not to get the Bondo too thick of it will crack with expansion over time.

As for your hardener, being in Tampa it's pretty warm so mixing it "Hot" (More Hardener) cuts you working life but you have already figured that out.

Be reminded I'm an engine tech not a body guy so another member with pro experience may chime in like @gmanpaint .

 

[Mbrooks420]

If you use too much hardener I’ve heard it can make the Bondo more brittle and likely to crack. I weigh mine out on a scale on the rare occasion I use it.

I’m much more likely to use fiberglass resin and mats.

 

[Mr. Alligator]

Why not just run back over the top lightly with 220 to rough it up a little? You need something for the next coat to stick to.

The trick is not to get the Bondo too thick of it will crack with expansion over time.

As for your hardener, being in Tampa it's pretty warm so mixing it "Hot" (More Hardener) cuts you working life but you have already figured that out.

Be reminded I'm an engine tech not a body guy so another member with pro experience may chime in like @gmanpaint .

Agreed about the advantage of coarse sanding for mechanical adhesion. Right now I have many small overlapping batches sitting on 400 grit sanding. Small and overlapping because I had reduced cure time with hot air temperatures and higher levels of hardener down to just a few minutes. And I just kept adding small piece to small piece with that technique. I figured I did not care, because it was getting done and it was entertaining.

Future batches for final coat will be more roughed up, and with less hardner to increase cure time and increase amounts of Bondo I could apply.

QUESTION: Does anyone think I should grind off the smaller batches to start over?

If you use too much hardener I’ve heard it can make the Bondo more brittle and likely to crack. I weigh mine out on a scale on the rare occasion I use it.

I’m much more likely to use fiberglass resin and mats.

Do you need horizontal surfaces for the fiberglass and mats application?

Thanks again!

 

[Mbrooks420]

Agreed about the advantage of coarse sanding for mechanical adhesion. Right now I have many small overlapping batches sitting on 400 grit sanding. Small and overlapping because I had reduced cure time with hot air temperatures and higher levels of hardener down to just a few minutes. And I just kept adding small piece to small piece with that technique. I figured I did not care, because it was getting done and it was entertaining.

Future batches for final coat will be more roughed up, and with less hardner to increase cure time and increase amounts of Bondo I could apply.

QUESTION: Does anyone think I should grind off the smaller batches to start over?



Do you need horizontal surfaces for the fiberglass and mats application?

Thanks again!

I’ve done lots of vertical work with fiberglass. Makes for a much more rigid and flexible repair.

 

[Mr. Alligator]

I’ve done lots of vertical work with fiberglass. Makes for a much more rigid and flexible repair.

I bet you get great results. Well done. I would probably have more sag, and mess.

 

[Mbrooks420]

I’ve always thought it was fairly easy to use. Definitely tougher sanding, though.

 

[MONMIX]

I would remove it all and go back over it with at least 180. If it is 1/4 inch thick ( which is right on the line of too thick) I would use 80 grit. Make sure all the paint is gone, body filler should never go over paint. Glaze ( top coat) can though as long as the paint is not shiny. Sand it in 80, then 180 when it feels just about right and then finish everything in 220 for primer.

 

[MONMIX]

I’ve done lots of vertical work with fiberglass. Makes for a much more rigid and flexible repair.

Rigid AND flexible?
:scratcheshead:

 

[shucker1]

Rigid AND flexible?
:scratcheshead:

Along side the dictionary by Government Intelligence?

 

[Mbrooks420]

Rigid AND flexible?
:scratcheshead:

I guess rigid isn’t the right word. Durable and flexible, then.

 

[MONMIX]

I guess rigid isn’t the right word. Durable and flexible, then.

makes MUCH more sense.