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Thought I’d post up my effort at resurfacing the quarter windows on my 90. I’ll first start off by saying I’ve never messed with Bondo and have no experience with body work. I’ll show my mistakes and things I had trouble with, I hope this helps someone else who wants to give it a shot.
Before:
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As you can see, mine were pretty rough. I chose to use the Bondo bumper repair kit as it’s supposed to flex a bit.
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I started by taping off all the surfaces around the rubber and sanding the rubber with 80 grit to rough it up (more).
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Once sanded, I mixed up the Bondo. This kit came with 2 packets of filler and 2 packets of hardener. I found that once they are combined, the working time is relatively short before it turns to cement. I would advise AGAINST mixing both packets of each at one time, I think it’ll become unusable before you get through it all. I then applied the Bondo to the quarter window and smoothed it out the best I could with the plastic spreader they give you.
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Like I said, this stuff dries quick, so work fast. It doesn’t give you much time to spread it and smooth it out nicely, I kind of just had to throw it on. But the quick drying time is nice because you don’t have to wait hours to start sanding it down.
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I started with 80 grit to really knock it down and then went finer and finer. It knocks the sheen off so you can see what you’ve sanded and what you haven’t. Be careful with the sanding here. If you remove enough Bondo to reveal the actual rubber, you’re gonna start sanding Bondo quicker than the rubber. This will create high/low spots on the finished product. I did it, and if I wanted to invest more time and money, I’d would have gone and gotten another bumper kit to add more Bondo. This being a “test”, I let it ride.
One of the trickiest parts are the edges. The Bondo spills over the edges and onto the tape, completely covering the edges of the rubber. I tried to be careful while applying it, but again, with such a short working time, I really couldn’t waste time being careful. I had to cut the edges along the rubber with a razor, remove the excess Bondo and sand the edges smooth. It wasn’t easy. I did the best I could. I got it smoothed out for the most part, and to a level that I thought would create a relatively smooth final product. Time to paint. For this I masked off the entire area and went to town with “Trim and Bumper” paint.
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Not sure how many coats I applied, maybe 5. Let it dry and unmasked. Here is the final product and a direct comparison between before and after.
Before:
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You can definitely see some surface imperfections, most were caused by the rubber starting to peek through. A little more filler would have been nice, but the kit is fairly small and some was wasted because it dried out. All in all, I’m pretty pleased. I can’t say they look new, but they have gone from a rough set of quarter windows to a decent set, for about $50. Someone who is better with body work could absolutely get better results. A thicker layer of Bondo could have gotten better results. A little better sanding job would’ve helped too, but hey, there’s always next time. Hope this helps someone who is contemplating this project, became we all know a new set of quarter windows ain’t cheap.

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1990 Ford Mustang LX 351M powered!! Project Cherry Bomb!!
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Great post and thanks for the info and tips!! Looks a lot better than before!! Good job!! :)
 

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Discussion Starter #3
Thank you. I’m gonna do the other side tomorrow and try to fix some of the errors I made on the first one. I may try mixing up only half a pack of filler/hardener at a time. If it turns out a lot better I may go back and redo this one.
 

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You dont need to mix the entire contents of each package together. You can do it in smaller and more manageable sections. You can also use less hardener and it will give you a longer working time. While drying and it is just tacky you can use a plastic razor blade to cut the edges clean so you dont have to rely on sanding the edges clean.

The manufacturer did not recommend applying any primer before going in with a top coat? This will give you an added step at refining any minor sanding scratches left behind. Once primer is dry you can wet sand that for a smoother finish before applying your top coat.
 

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Discussion Starter #5 (Edited)
@87gtVIC So I took your advice on the second side. Mixed less bondo at one time, which allowed me to go back after sanding and add more to the low spots, then primer, then finish sanding, then paint. The good news is it looks substantially better. The bad news is it looks like I’m redoing the first one. Thanks for the advice, it really looks better. Still not perfect, but much better.
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Body work takes time and lots of it. Even more time if you are new to it an want good results.

You will get even better results this next time and you will want to redo the other side as well. Practice. You can only get better with it.
 

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Discussion Starter #7
😂😂😂 I’ve started a vicious cycle. I’m no master with anything related to cars, but I’m also not afraid to try stuff. It’s going to lead to some flaws, but I figure doing as much as you can on your own is half the fun of having a project. I really appreciate your input, I’ve already seen the improvement. And I’m sure you’re right, practice makes progress. Thanks again🤘🏻
 

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That looks like an epoxy based filler so it needs to be mixed 1 to 1 with hardener. Body filler can use less or more hardener but not epoxy.
 

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Discussion Starter #9
I did mix 1 to 1 (or as close to it as I could estimate). I just mixed smaller batches so I could get through it all before it hardened.
 

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You dont need to mix the entire contents of each package together. You can do it in smaller and more manageable sections. You can also use less hardener and it will give you a longer working time. While drying and it is just tacky you can use a plastic razor blade to cut the edges clean so you dont have to rely on sanding the edges clean.

The manufacturer did not recommend applying any primer before going in with a top coat? This will give you an added step at refining any minor sanding scratches left behind. Once primer is dry you can wet sand that for a smoother finish before applying your top coat.

That looks like an epoxy based filler so it needs to be mixed 1 to 1 with hardener. Body filler can use less or more hardener but not epoxy.
Oh shoot. Thanks for pointing that out. It does look like an epoxy.
 

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Mine were not as bad, I just used a rattle can of high build primer. I did 2 or 3 coats and then sanded smooth and painted. A little adhesion promoter would not hurt either before the primer.
 
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