Advice for applying Bondo

"zeontestpilot" (zeontestpilot) 10/30/2015 at 08:00 • Filed to: Homelopnik

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I’ve been going headstrong into a bunch of house fixes lately, and eventually it’s gonna be this guys turn. What you see here is the metal side panel next to the front door. The outside has already been fixed by gramps, who’s now in Florida enjoying whatever they enjoy [probably hurricanes]. So it’s up to me to fix the inside part.

Now, what gramps did, which I plan to imitate, is take a wire brush to it to scrap off the excess pieces, and then apply bondo (with the hardening agent, or whatever it’s called), and apply it to the ‘wound’. I understand there’s a time limit until it hardens, so I’m trying to be prepared with a plan of attack.

So...anything I need to know, or advice you’d like to give me? I’ve never done bondo before, so it’s all new to me. I watched a few videos online and have a rough idea, and I have all the necessary tools (x2 putty knives, cardboard to mix bondo on, towel to clean knives, etc) and the materials (bondo, hardening agent). Am I missing anything? Any tips or tricks? Anything can and will be helpful!

Here’s another pic, farther away.

Master Cylinder > zeontestpilot 10/30/2015 at 08:11

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Sounds like you’ve pretty much got the basic idea. Spread it on no thicker than necessary; it’s better to do multiple thin coats if you’re trying to fill a deep void. Let it cure completely before you sand. It’s pretty easy to work with, especially when you don’t necessarily need a perfect finish.

450X_FTW > zeontestpilot 10/30/2015 at 08:13

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for every golf ball size of bondo, use one inch of hardener to mix together. Mix until they are one color. I always use a piece of cardboard to mix on since it’s going in the trash when your done anyway. Don’t be gentle, slap it on and spread it as evenly as possible. Start with 36-40 grit sandpaper. I’d wrap it around a block of wood so you have a straight level surface your sanding against and it’s easier on the hands. Don’t sand it too early otherwise it’ll come off in chunks. Don’t wait too long otherwise you’ll need a power sander. Go in an X pattern when sanding so you don’t create grooves or lines in the bondo. Can wash the dry bondo off your spreader using paint thinner. Plastic spreaders are your friend. Most of the dry bondo will crack off the spreader

BigBlock440 > zeontestpilot 10/30/2015 at 08:16

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The smoother you apply it, the less sanding you have to do.

nerd_racing > zeontestpilot 10/30/2015 at 08:16

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Make sure you don’t let it get rock hard before you sand, let it harden just enough that the dust doesn’t stick to it before you start sanding. just hard enough that your thumb nail doesn’t dent it. If you let it get rock hard it will be much harder to get smooth. Pro tip I learned from the guy who ran a local body shop after I botched the bondo on a pickup I was trying to restore in highschool.

zeontestpilot > 450X_FTW 10/30/2015 at 08:17

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Is there a way to tell when it's just right to sand?

450X_FTW > zeontestpilot 10/30/2015 at 08:18

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If you touch it, and it’s not stringy, doesn’t deform like puddy. Been a while, but I want to say never wait more than 30 minutes

zeontestpilot > Master Cylinder 10/30/2015 at 08:18

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It's going to be painted in the future (not my department). If it's cured completely, would I need a power sander then?

CalzoneGolem > 450X_FTW 10/30/2015 at 08:21

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Those spreaders also make excellent dough knives.

Meatcoma > zeontestpilot 10/30/2015 at 08:24

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If you can take the door off and put it on some saw horses it will be much easier to work with.

I have no tips for bondo so I offered what I could.

zeontestpilot > 450X_FTW 10/30/2015 at 08:26

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How long would you say it takes for it to be cured completely? So I can apply a 2nd if necessary.

Bandit > zeontestpilot 10/30/2015 at 08:28

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First off, I’d suggest buying a finishing putty (Like Evercoat Gold) rather than bondo, it dries in about 5 minutes as opposed to 2 hours for bond0. Second make sure all the rust is cut out or neutralized. Then follow everyone else’s spreading tips. Try to use a block when sanding it flat. You will need more than one coat. Thin coats dry nice and quick while thick coats take forever. If you really want to get fancy, use different color hardeners to differentiate between layers.

Master Cylinder > zeontestpilot 10/30/2015 at 08:29

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Not really, Bondo doesn’t cure super hard. It’s roughly similar to hardwood in terms of sandability. I’d use a power sander if I already had one, but I wouldn’t buy one just for the purpose.

Master Cylinder > Master Cylinder 10/30/2015 at 08:31

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Although based on some of the other comments, maybe you don’t want to wait until it’s completely cured. I’m certainly no Bondo expert.

zeontestpilot > BigBlock440 10/30/2015 at 08:32

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Ha, somehow I didn't think of that, thanks! I'm trying to plan multiple projects at the same time, :).

450X_FTW > zeontestpilot 10/30/2015 at 08:46

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I’d given it an hour, Just remember to sand between layers otherwise it will get far too thick. If you have a long level, place it on the door and drag over the bondo so you can find your high/low spots.

BigBlock440 > zeontestpilot 10/30/2015 at 08:55

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Sanding is a pain, especially if you’re doing it by hand. If you have an electric sander, it’s not too bad though. It should also be easier on a flat surface like you’re doing, rather than having to try and match the contours of a car body.

Also, if the holes are really big, you should use a backer material. I use a piece of metal screen, but you could probably find other stuff to use. The goal is to have material on there to apply the bondo to, so it’s not just a lump you’re trying to build on top of.

BigBlock440 > zeontestpilot 10/30/2015 at 08:59

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Apparently it’s easier to sand before it’s fully dried. I guess that’s why I always hated it, I let it dry completely.

zeontestpilot > Meatcoma 10/30/2015 at 09:01

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It's too cold todo that, :(.

Meatcoma > zeontestpilot 10/30/2015 at 09:08

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Leave it til spring!

zeontestpilot > Bandit 10/30/2015 at 09:09

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Gramps bought me bondo, so it’s free, :).

So cut out the rust? Would that mean that I'd have to fill the entire wound with bondo then?

OCD-CO > zeontestpilot 10/30/2015 at 09:26

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Given the time of year it will not harden as fast. Just an FYI. I learned about Bondo pretty quickly last summer in a 95 degree day. Hardened before I could even apply it.

But, I am thinking that this is not your answer. I am not sure how handy you are, or if you are driven for perfection, but I would take that stripping off and pull that window panel and do it properly. You may find it easier to do this way as well.

If it were me I would be looking to replace. Find the panels/windows and put in new. Way less time and a better fix.

But,.............. if ya wanna learn how to use Bondo on this project, have at it! It is even more fun on a body panel.

Also, rust that deep and bubbly is a strong indicator of metal failure as well. I just looked over the pics again and I would not just Bondo those. There is a lot of water damage on that wood base board as well. I would pull all of that and check - you may have a rotted floor as well.

Stapleface-Now Hyphenated! > zeontestpilot 10/30/2015 at 09:35

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Seeing as how it’s already rusted once, might it make sense to hit it with some type of rust preventative (like POR-15) first? Is there good metal under the rust?

On a (somewhat) unrelated note, can't say I've ever seen a metal sidelight.

zeontestpilot > BigBlock440 10/30/2015 at 09:36

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Hmmm, so I need a backer material then?

zeontestpilot > BigBlock440 10/30/2015 at 09:37

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Hmmm, so I need a backer material then?

Bandit > zeontestpilot 10/30/2015 at 09:47

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If you dont neutralize or cut out the rust, it will simply come right back.

BigBlock440 > zeontestpilot 10/30/2015 at 09:49

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It might help, depends on how much filler you want to go through and how thick you make it. If it starts to harden, it’ll be easier to build up but it’ll be harder to sand.

zeontestpilot > Meatcoma 10/30/2015 at 09:49

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I’d imagine adding bondo would help insulate s bit, right?

zeontestpilot > Bandit 10/30/2015 at 10:09

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Wasn't aware rust operates like that.

zeontestpilot > Stapleface-Now Hyphenated! 10/30/2015 at 10:18

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I haven’t checked further than the surface. So I'll have to do that. Is it called a sidelight?

Bandit > zeontestpilot 10/30/2015 at 10:24

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Yeah, I mean, it may take half a year, but if imperly treated it can actually push the bondo right off the door. I had to make proper metal patch panels for my last truck because the PO bondo’d right over rust and the panel was falling apart from the inside out. But, you’re working on a door, not a car. So your standards for quality are probably slightly lower. Pictured: Rust found under previous owners bondo job.

Stapleface-Now Hyphenated! > zeontestpilot 10/30/2015 at 10:26

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Yup, called a sidelight. Most of them can be replaced individually if you need to go that route. From the looks of the framing around the door, that appears to be the case with yours as well. I recently kicked in my front door accidentally and busted my door frame. Unfortunately for me mine was all one piece. But yours thankfully doesn’t look that way.

uofime-2 > zeontestpilot 10/30/2015 at 10:26

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Definitely pop this trim off first and than mask everything off, if you get a bunch on your masking, go ahead and remove the masking before it dries, and then reapply once it is.

bondo should never be more than a quarter inch thick.

Too much hardner is far better than not enough. trying to remove uncured bondo is a nightmare

If you’re using genuine bondo, with red hardner, the material should be light pink when you apply it.

expect 10-15 mins of work time

once it starts to clump and won’t spread smooth its done, throw away whatever is left

work in small batches

use thin coats, use even reasonable pressure on your spreader, air bubbles are your enemy, you’ll have to sand the whole coat down if you get them.

you can kind of practice spreading on your cardboard while you mix.

Wear gloves its always messy

Keep the room as ventilated as possible, keep kids, pets away. The MEK in the hardner is nasty stuff.

Don’t worry, it’s pretty hard to irreversibly mess this up. You can usually always just sand it off and try again. Of course sanding it off is a pain, so you will be well motivated to improve your skills as quickly as possible!

Snuze: Needs another Swede > Stapleface-Now Hyphenated! 10/30/2015 at 10:31

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I hope there’s a good story here...

Stapleface-Now Hyphenated! > Snuze: Needs another Swede 10/30/2015 at 10:35

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No, it's me being a momentous dumbass. I was coming in the front door carrying something. The door began to close and I kicked it trying to keep it open. But it latched first, and I blew the door frame right out. I chalked it up to a shitty builders-quality door that was put there when my house was built in 1999. If it was an old house, I bet nothing would have happened.

zeontestpilot > OCD-CO 10/30/2015 at 10:37

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The issue with new is that we’re severely lacking with funds, and we’re opting for cheaper fixes.

Whenever it snowed, the previous owners just let the snow sit there, and entered the house via the backdoor. Also, this room, the living room, is the old garage. So it’s a cement slab underneath everything. The front door is where the garage door used to be. The outside has been waterproofed/sealed too. Not sure if it was originally, but it is now.

I’ll have to look at it once I get home, but the wood damage is something I didn’t consider. So thanks for the heads up there.

Snuze: Needs another Swede > Stapleface-Now Hyphenated! 10/30/2015 at 10:43

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Gotcha. Yeah, they just don’t build stuff like they used to.

I once locked my keys in my house when I was living in CT. I had just met Mrs. Snuze at the time, she was living in MD, and I was on the phone with her when I realized I locked myself out. I spent a solid 20 minutes trying to kick the door in to no avail. I had to call a lock smith and pay $100 to get back into my own house.

A few weeks later we were talking and she mentioned her uncle lived in New London (next town over) and that he’s a locksmith. She’s lucky the relationship didn’t end right there.

Meatcoma > zeontestpilot 10/30/2015 at 11:12

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SURE!