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Filling dents in steel frame with MAPP

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Filling dents in steel frame with MAPP

Old 05-28-22, 01:32 PM
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Sahn
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Filling dents in steel frame with MAPP

I have several steel bikes with minor dents I want to fill before repainting. I have a big-box MAPP torch and very little experience - a recipe for disaster? or is this a DIY'er project? Besides bondo - any suggestions? THANKS!
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Old 05-28-22, 02:03 PM
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JB weld. it'll stick forever. You can sand it, its easy to work with and its cheap
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Old 05-28-22, 02:28 PM
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A big reason to use a metal filler for dents is if you plan to powder coat the frame. While I do believe there are epoxy like fillers that withstand the heat level to cure the powder coat they are not typically found in your local hardware store.

Depending on how deep the dents are a low temp solder, 56% silver braze or brass/bronze are the usual fillers. I strongly a practice or 4 as getting the temp level just so for the filler to melt but not run off can be tricky for the beginner. If the paint will be a wet spray on then I suggest bondo/JB Weld. Andy
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Old 05-29-22, 06:00 AM
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Hello all,

I have a similar need to fill a dent prior to painting (powdercoat). I have worked with a local framebuilder nearby before, but he is unavailable until the fall. I brought the frame to an auto body shop, the owner will give it a try, but it just occurred to me that the dent is very close to a top tube cable guide, is there a risk that the heat could cause the guide to come off?

Thanks,
Roger

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Old 05-29-22, 06:49 AM
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I've taught framebuilding classes for many years. What I know is that there is a long learning curve to using either oxyacetylene or oxypropane torch well enough to do the tasks both of you have asked about. Knowing what I know, the chances of you guys being successful is very small and the chance you seriously damage your frames large. Good brazing takes a delicate touch. There is absolutely no way whatever I would let some auto body guy experiment on a frame of mine.

I"ve also filled many frame dents with both brass (actually bronze) and silver. It is so easy if you are not experienced to not do it right. 1st of all I would never try it with a Mapp torch because its lower and broader heat output will expand your puddle too much. This job requires instant reaction or the filler runs away. This reaction time is learned by experience and almost none of my students could do this job successfully even with the right brazing equipment until after many hours of practice. It takes awhile for their recognition of what they need to do and their hands to follow immediately.
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Old 05-29-22, 06:49 AM
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Yes there's a risk that the person who is handling the torch won't pay enough attention to where and by how much the heat is applied. A skilled brazed should be able to fill this dent w/o affecting the guide. Andy
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Old 05-29-22, 07:30 AM
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Copy that, I'm glad I checked. I called the body shop and cancelled the job. Does anyone know of a frame builder in the Boston/North Shore area that could do this?
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Old 05-29-22, 01:17 PM
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There are high spots around the dent that cannot be removed by adding filler, so they will always be high spots. Unless you try filing them down, in which case you'll go through the tube wall and have a hole in the frame. Moral of the story is don't try filing down the high spots, not even a little.

I would remove the BO, and roll the dent down with metal tube blocks (I use steel but aluminum works too), which takes the high spots down a lot. Not all the way, but usually close enough to look decent in paint. The low spot also usually pops back out at least some — still a low spot, but less filler needed. Rolling out dents requires skill too but it's probably the least-dangerous part of the repair. Most people who own the metal tube blocks have done it, and so won't be complete newbies. If you want to try that yourself, ask for tips here, I think a newb can handle that step with some advice. Maybe there's a youtube on it?

That BO is far enough from the dent that a good FB could fill the dent with the same silver that would then be used to put the BO back on, without re-melting the dent fill. Of course any filing/sanding of the filler would be done before re-installing the BO. The filing also needs to be done by someone with a steady hand. If you tip the file a little this way or that, in one stroke of the file you've put in a nick that can be a significant portion of the tube wall, if it's a lightweight. That's a stress-riser and the frame could fail there in fatigue later. The thin unbutt of a quality frame can be as thin as 0.4 mm (.016") but yours is probably more like .6 or .7 mm. Still thin enough to require care and some skill. My advice is try to add the bare minimum of filler, to minimize the amount of filing needed.

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Old 05-29-22, 03:50 PM
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It's Reynolds 531c (circa 1983) if that makes a difference
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Old 05-30-22, 07:01 AM
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So... for the novice DIYer for wet finish it's: Sand, roll, prime, epoxy fill, sand, prime, paint and clear coat? Beyond rolling... should an attempt be made to further reduce high spots with a body-work hammer? Any suggestions for primers on bare metal and over bondo would be appreciated - as well as any further insights.
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Old 05-30-22, 12:26 PM
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Originally Posted by Sahn View Post
So... for the novice DIYer for wet finish it's: Sand, roll, prime, epoxy fill, sand, prime, paint and clear coat? Beyond rolling... should an attempt be made to further reduce high spots with a body-work hammer? Any suggestions for primers on bare metal and over bondo would be appreciated - as well as any further insights.
if you are going to do all of these, best to use automotive paint if you can as it is far more durable than "rattle can" spray paint.

Depending on where you live, you can get auto paint packaged in a spray can from a auto paint dealer. and some activated clear is available that has a button to push on the bottom of the can to activate

I used auto paint an a preval paint sprayer https://preval.com/

Acid etch primer for bare metal, followed by sand able primer

pay close attention to respray times

and of course good mask at all times
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Old 05-30-22, 04:03 PM
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Originally Posted by squirtdad View Post
and of course good mask at all times
For anyone reading this who doesn't know, "a good mask" doesn't mean a covid or dust mask, it means as a minimum a cartridge respirator, with proper cartridges for organic vapor. New cartridges because their ability to absorb the bad stuff gets used up. Seal them up when you're not using them so they aren't absorbing stuff from the air, getting used up to no benefit.

Note I'm no expert, not a painter at all, so get respirator advice from someone who knows. I think pros use an air-supply respirator, with the air pumped into your hood from somewhere far away from the nasty chemical cloud. A cartridge respirator is probably OK for occasional use though.

If you have a beard, it's likely that no respirator will seal well enough against your face, so you might want to consider a hood.

My info is decades old, from when I worked in a shop that had two full-time painters. Back then the chemicals they were using were crazy bad for you, like you don't even take the lid off the can without your respirator on.

Maybe that's not true for all automotive paint. None of it is benign though.

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Old 05-30-22, 08:13 PM
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They have been using isocyanates since the '70s, and apparently there is nothing better they can use in urethanes. It's bad stuff. charcoal filters are needed at a minimum, and they need to be replaced fairly often. That's why pros use supplied air
It's also why I don't paint right now. Isocyanates can give you asthma, and my asthma doesn't need any help.
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Old 05-30-22, 09:26 PM
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Living with the dent is another option...
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Old 06-05-22, 11:42 AM
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Originally Posted by Sahn View Post
So... for the novice DIYer for wet finish it's: Sand, roll, prime, epoxy fill, sand, prime, paint and clear coat? Beyond rolling... should an attempt be made to further reduce high spots with a body-work hammer? Any suggestions for primers on bare metal and over bondo would be appreciated - as well as any further insights.
Yes use car body filler. If you're painting the whole frame I usually use Montana Gold acrylic, Montana Gold steel primer and glossy 2K clear. Actually a pretty decent finish. Leave the acrylic to dry fully (like a week indoors) before putting on the clear and be advised that the clear doesn't reach full strength for maybe a month so be careful when building up the bike.
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Old 06-18-22, 07:33 AM
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Originally Posted by Andrew R Stewart View Post
Yes there's a risk that the person who is handling the torch won't pay enough attention to where and by how much the heat is applied. A skilled brazed should be able to fill this dent w/o affecting the guide. Andy
Thanks to all for the advice, I brought the frame to Peter Mooney at Belmont Wheelworks, got new cable guides as well, came out great.

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Old 06-18-22, 12:05 PM
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Originally Posted by rsacilotto View Post
Thanks to all for the advice, I brought the frame to Peter Mooney at Belmont Wheelworks, got new cable guides as well, came out great.
Yep that's a top-notch job. Thanks for posting the happy ending.

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Old 06-18-22, 01:55 PM
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Can't do much better than Peter Mooney. Thanks for the follow up. I can well understand why Peter wanted to replace all 3 guides. Between having a match to the old and the easier to deal with new this is a minimal,brainer. Andy
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Old 06-18-22, 02:24 PM
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Originally Posted by guy153 View Post
Yes use car body filler. If you're painting the whole frame I usually use Montana Gold acrylic, Montana Gold steel primer and glossy 2K clear. Actually a pretty decent finish. Leave the acrylic to dry fully (like a week indoors) before putting on the clear and be advised that the clear doesn't reach full strength for maybe a month so be careful when building up the bike.
X2 on the Montana gold, that stuff is beautiful to spray! not an option if you want a color match though. For that you'll be heading to a body shop supply place, they can scan the existing color and put a match in a spray can.


For me I only ever leave 24hrs dry time for the color, then i wet sand it then spray the clear. If using Spramax i leave it for 48 hours before i go to town on it.
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