Hot Rods best practices for bondo application?

How thick can the bondo be?
What do you do to keep the work area clean?
How often should I use degreaser?
How often should I change the sanding pad on the mud hug?

 

1/4 inch maximum is usually what the tech sheet says
Clean? Shops are supposed to be dirty.
Try a shop vac. Use a fan to suck the dust out as you sand
clean with degreaser before you start the repair. Clean before you prime. I don’t like to soak the filler with cleaner. I’ll wipe the perimeter around the filler. Make sure the cleaner evaporates before priming.
Change paper when the paper stops sanding

 

1/4 is awfully thick in my personal experience. If I absolutely had to fill something that deep and couldn't metalwork it closer than that, I think I'd use short strand fiberglass first, rough it in and then a skim of filler over top. I feel like 1/8" or less is body filler range.

Freshly sanded steel should take filler directly without the need for a degreaser. I don't use degreaser during body filler, hasn't been an issue.

As for sanding pads, that's up to you. If they are getting gummed up, you probably need more cure time. Otherwise, like Anthony said, swap them out when they stop sanding efficiently.

I change DA pads pretty often, because I value my time more than the cost of a sanding disc. But a guy I used to work for, liked to get every last mile out of sanding paper, used them until they were smooth.

 

Yep. 1/4 is a lot. Seen it stacked a lot deeper than that
Just going off tech sheets I’ve read.

best practice is work the metal as good as possible.
But sometimes ya just gotta do what ya gotta do

 

K13 works for one of the companies and did this thread a while back. Lots of good info.
Technical - Body Filler Basics | The H.A.M.B.

 

My brother did bodywork on the side, at home, for a few years.

He took in a 68 or 69 Porsche 911 to do some work. I was there one day and saw it. He was asked to do some small things in the front of the car.
He showed me about a 3 pound piece of Bondo, that he removed from a rear fender. The ONLY reason that he found it, he ran his hands up under each fender looking for old body work filler. On the paint finish, the piece looked...fine !!
NO CRAZING OF THE PAINT, NO CRACKS IN THE PAINT, NO UNEVENESS..!!
The Bondo at it's deepest was over...2-3/4" (maybe closer to 3") thick !!! The dent looked like someone was thrown at the car and an elbow put a deep dent in the right rear fender.
My brother didn't know how old that work was, but it looked to be at least a couple of years old, by the paint surface.

So...there you are. Done well, at least a 2-3/4" thick slab of Bondo CAN...be used.

Mike

 

Is it bad practice to use a mud hog to grind down newly applied bondo to get it roughed in? Also the paper side of the mud hog was sitting on the shop floor. So this may have introduced contaminants into the bondo. Should I be concerned? Also is it to late to weld on a panel when the other half of the panel has freshly applied bondo?

 

I never did much bondo work , I absolutely hate doing it . I try my best metal finish , cut out weld in new metal to keep the filler as thin as possible . When applied correctly it usually will out last the top coat with no issues . I was taught to apply to a roughly prepped panel full of deep sand scratches the filler will take care of the scratches and it gives the fillers something to bite into and stick to . I have been told to prime fist and others say only bare metal , I do not know the correct way to apply the 1st coat of fillers .

 

Body work is a learned experience, the more you do it, the better you are suppose to get at it. Its kind of like welding in that regard, few are great right at the start, some get much better every time you do it because practice improves the process, but some people never quite reach the range of being great at it.

A 1/4" maximum thickness is a great goal, but the reality is, once the job is done, you really have no idea how thick it really is, unless to remove it. You can grasp that the filler is getting too thick, but at that point, there isn't much you can do about it, short of removing it all, and starting over with better metal work. So you keep plugging along, but remember to do better metal work the next time.

It generally doesn't take too long before you understand what wastes sanding paper and what doesn't, and you automatically refrain from doing those things that waste the sanding paper, its part of the learning process. Buying more sanding paper is traditional, I think.

For me, the hardest part of body work is to know when to quit sanding! I often find myself refilling the same areas a few times because I wanted it smooth, but sanded too much. My personal tolerance for doing body work has a pretty short time span. I know how to do it, but I lack the patience to take the time to finish it correctly. Every thing I do body work on tends to be a friendly vehicle, it waves at everyone as it passes by. If that bothers someone, feel free to take your concerns to my body guy, he is always close by me.

 

Not to play devil's advocate, but the Bondo tech sheet could be read to not apply the product that is more than 1/4" thick per layer.

 

I once watched a guy dump a gallon out on a hood, squeeze in a tube of harder and spread it all out. The same guy stuffed newspaper into rust holes and spread the hod over it.

 

Some of the filler products can hold moisture and cause rust if applied directly onto the bare metal.
there are some that don’t
That’s why it’s very important to read the technical data sheet

 

Remember he said Suck! Not blow! Big difference.

I cringe at the thought of burying bare metal under a coat of body filler. I know that not all products attract moisture and people have been doing it this way blah blah. It takes two minutes to treat the steel with some metal prep so why not? Admittedly I haven’t done much automotive body filler work in 35 years however I served in the Air Force and did a LOT of corrosion repairs so I’ve seen what a little trapped moisture can do.


What is a mud hog?

 

Absolute, Most important factor to me telling someone that doesn't do it regular, Use Sharp paper...
Change often, An don't be afraid to sand past the dime size ding when doing the repair.

 

8 inch DA sander

 

The issue I’d have of welding on a panel after you started filler on the other end is with warpage. Any welding will introduce warpage to the panel, and then you need to remove/hammer it out. If this work gets near the fresh bondo it might make it flake loose. A lot of that possibility depends on how good the adhesion is (grind the panel before applying the bondo?).

And if you weld right next to bondo, it will burn off.

 

Been there.
Wiped it with a car tag

 

A roll of 1/8" screen and a couple gallons of body putty and you can about build a whole car!
I have seen old cars and trucks that have been sitting outside for years with body putty 2" thick on them and you still need a hammer to get it off. Not saying it is correct, but it got the job done.

 

Remember it shrinks, and keeps on shrinking. The thicker it is the more it changes shape. So if it's a critical, and very visible body line, keep it as thin as possible. It is extremely porous so absorbs water. Any chance moisture can get in it will just stay there under the paint.

 

I must have bought his truck about 15 years ago.

 

If I touch the metal before applying bondo should I use wax and grease remover?

 

I’ve never worried about that.
I try to wear gloves when the ride is in bare metal

 

I’ve had dozens of “ winter beaters “ over the years .

some where more bondo , mono foam , and that short strand fibreglass/ bondo concoction then actual steel !!
I would make “ forms “ out of duct tape and cardboard for fender lips then sculpt in the bondo .

now , these where cars that had to serve a purpose, 3-5 months of service then either sold for cheap or scraped or left in a parking lot because of some mechanical failure .


Now , on a car I actually cared about and would be proud of the work that I put into it , try to keep it at an 1/8th or less filler in any given spot .


With saying that , if the underlying metal , work performed , etc etc is of good quality , but your metal finishing skills are simply lacking . Then do what you need to do. No shame or embarrassment in knowing what your limits are . And let’s face it , professional metal work , #1 to find a shop that will do it , and #2 the cost involved in paying someone can be ridiculous.

 

Depends what's on your hands. If dry and clean, nothing to worry about. If greasy or sweaty, then yes.

I've never had a problem myself, as I have very dry hands.
I had a neighbor kid come and help me strip the paint on a car. His hands were always sweaty. Everywhere he touched bare metal on the car with his hands, was rusty 2 days later. Literally rusty finger and handprints. Not an issue anywhere I touched.

 

Todays fillers are a different animal long gone are the fillers that are slapped on bear metal there maid to go over ecoat or primer that doesn't mean you can't do it but I found some fillers do peel back and nothing pisses me off more then that time is money. As for using a DA to nock down mud don't block it use guid coat if needed I work with a 40 year tech that always uses a DA to nock down mud never blocks and almost always gets his work back from the painter because of waves.

 

This is exactly how I feel. I'm assuming that OP cares about whatever it is he's currently working on, otherwise these issues wouldn't be any concern. But I too have boogered up some rough beaters just to get a few last miles out of them, and that's where anything goes.

And really, unless you're covering up structural issues (which I have definitely seen), do what you have to do to get a car finished. A car that's done and driving is always better than one that is theoretically going to be perfect if it ever gets done (which most do not).

But that said, the less you use, the more likely it's going to hold up. And if I know someone has touched a bare metal area before I got to put filler there, I like to run over it again with a sander. Wax and grease remover is a good product and has its uses, and would probably do a good job at getting rid of fingerprints before body filler, but if you're not careful, it'll leave a film, and then you're worse off than when you started. Used properly, if you're really getting the area clean, it'll do a nice job, and not hurt anything, but I don't know if it's necessary.

You might be over-thinking it.

 

I usually just blow off the panel really well before mud. I've found if it's sanded properly that'll get it more than clean enough. And there are hole sections of filler I've worked in with a 6" da. 180 grit before prime and 320 on a finish da before paint. I try to keep it as thin as possible. And will not bondo over a hole or rust. They'll be spots on my newest project 54 where the filler is thicker than I like but all metal will be solid underneath

 

When I bought my 49, both front fenders had a layer of bondo on them. It looked like someone had sanded them smooth (they looked pretty good) except the fenders were left out in the weather and what was at one time smooth bondo had pits in it from water sitting on them. Obviously, that had to come off.
I started with some pretty course paper, it just didn't seem like I was getting very far, but the pile of dust on the floor was getting thick. I stepped up to a grinder (that made a bigger mess real fast) but I still wasn't reaching any metal. I drug out the air impact gun with a chisel bit on it. That was when I discovered nearly all of both fenders had at least a 1/2" of filler covering the metal! When I finally got all the old bondo off the fenders, they were not really that bad! I have no idea why someone put so much filler on both fenders. When I got to the nose piece that bolted to both fenders down to the metal, I knew why it had a lot of filler piled on it. It was really rough. There was also a section on the roof where the curved area was bent downward almost 3". The bondo artist had the entire crater filler with bondo, it was sculptured very nicely. But after the fenders and nose piece experience, any bondo patch on the entire truck was faced with skepticism, most of which was deserving.
There is probably more filler in some places on the truck now then there should be, but there is good metal under the filler. I intend to keep it until I die, and I don't care if the filler may be more then a 1/4" thick in some places.

 

metal finish and lead...