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Is bondo needed after plug welding the holes?

PostPosted: Sat Jan 23, 2021 1:34 am
by TheWhopper
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!

Re: Is bondo needed after plug welding the holes?

PostPosted: Sun Jan 24, 2021 8:28 am
by '68 Coronet R/T
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.

Re: Is bondo needed after plug welding the holes?

PostPosted: Wed Jan 27, 2021 7:57 pm
by slednut
I use one of these all the time when filling holes.
https://www.amazon.com/Durston-Manufact ... 358&sr=8-2

Re: Is bondo needed after plug welding the holes?

PostPosted: Thu Jan 28, 2021 9:50 am
by '68 Coronet R/T
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?

Re: Is bondo needed after plug welding the holes?

PostPosted: Sun Jan 31, 2021 11:58 am
by Randy Forbes
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.

Re: Is bondo needed after plug welding the holes?

PostPosted: Sun Feb 07, 2021 11:23 am
by slednut
'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.

Re: Is bondo needed after plug welding the holes?

PostPosted: Thu Feb 11, 2021 2:47 am
by Doright
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.