Is bondo needed after plug welding the holes?

More of an art than a science - discuss metalworking and welding here.



Non-Lurker
Posts: 1
Joined: Sat Jan 23, 2021 1:28 am

Country:
USA
PostPosted: Sat Jan 23, 2021 1:34 am
Newbie here! Thinking of buying me a used car and removing the side flares. Needless to say it will leave about 24 holes on each side of the vehicle. If I can plug weld the holes and sand it down to a smooth even finish, is bondo still needed? I figure I'd ask the experts before I attempt such an endeavor. Thanks!

User avatar

Board Moderator
Posts: 9016
Joined: Fri Oct 20, 2006 12:40 pm
Location: ARIZONA
PostPosted: Sun Jan 24, 2021 8:28 am
Not if they are welded properly. Once ground to surface level epoxy primer should handle any imperfections.
When needed you want filler to be a skim coat of 1/16" or less after sanding.

Getting your welder set properly is the key. Practice on some scrap metal until you get the weld to lay flat and level filling the hole.

If the holes are large you will need to cut metal to fit. In some cases using a copper backing plate will help.
1968 Coronet R/T


ACTS 16:31



Settled In
Posts: 37
Joined: Fri Jan 31, 2020 4:29 pm
Location: Washington State
Country:
USA
PostPosted: Wed Jan 27, 2021 7:57 pm
I use one of these all the time when filling holes.
https://www.amazon.com/Durston-Manufact ... 358&sr=8-2

User avatar

Board Moderator
Posts: 9016
Joined: Fri Oct 20, 2006 12:40 pm
Location: ARIZONA
PostPosted: Thu Jan 28, 2021 9:50 am
slednut wrote:I use one of these all the time when filling holes.
https://www.amazon.com/Durston-Manufact ... 358&sr=8-2


Looks like a great tool. Does it perform as expected? Is the magnet strong enough?
1968 Coronet R/T


ACTS 16:31



Non-Lurker
Posts: 3
Joined: Sat Jan 30, 2021 2:58 pm

Country:
USA
PostPosted: Sun Jan 31, 2021 11:58 am
First post/introduction, so please forgive a repetitive answer...

Plug, rosette, buttonhole welds (different terms for the same thing) typically leave a slight mound above the surface. This can be easily/quickly cut down to the just a few thousandths above the surface and finished off flush with a finer grade of disc (BIG FAN of using a 2" disc on a pneumatic 90* die grinder for this).

The copper backup is universally accepted when filling holes, but having some nice chunks of it around, drops from earlier projects, I find myself often putting it to good use.
Attachments
20210125110721-112329ea-me.jpg
1967 MGB GT battery box repair
20210125110652-449a280a-me.jpg
20210125110606-0761503f-me.jpg



Settled In
Posts: 37
Joined: Fri Jan 31, 2020 4:29 pm
Location: Washington State
Country:
USA
PostPosted: Sun Feb 07, 2021 11:23 am
'68 Coronet R/T wrote:
slednut wrote:I use one of these all the time when filling holes.
https://www.amazon.com/Durston-Manufact ... 358&sr=8-2


Looks like a great tool. Does it perform as expected? Is the magnet strong enough?


This is a must have tool, holds better than I expected.



Top Contributor
Posts: 5543
Joined: Tue Sep 16, 2008 1:17 pm
Location: Pahrump NV.
Country:
USA
PostPosted: Thu Feb 11, 2021 2:47 am
The use of a Cooper backing plate while accepted by many is a band aide of sorts in my opinion.

The Copper wont stick to the weld bead, and the Bead on top looks good like its all fused together But if you look on back side of the weld bead it isn't fully fused all together at least it wasn't when I tried using it on multiple attempts using a Copper backing plate. It looked liked it kinda laied out like dog poop with what looked like cracks and voids when looked at closely.
Whats more the Copper acts as a Heat sink drawing heat away from the bead requiring a Higher heat setting on the welder to get a good bead.

How your bead looks on the back side is more important than the Top side!
Yes both get ground down but if you have voids cracks and impurity's in your weld its a recipe for a come back or disaster.

Its better to cut and make small patches for filling holes.
Dennis Barnett
A&P Mechanic, FCC General radio Telephone Operator
Line Maintenance A&P Mechanic and MOC Tech specialist.

Return to Welding & Metal Fab

Who is online

Users browsing this forum: No registered users and 14 guests