Technical Body Guys - How to Adhere New Mud to Existing Mud

I"m expecting a lot of flack for this one BUT here goes.

Fender bender on one of my rides. It was a finished car when I acquired it. So it has existing mud of unknown pedigree in it already.

I need to mud over the pieces together that have been welded (as in new metal) and then transition into the rest of the existing filler mud.

No,I"m not taking the whole thing to bare metal. Call it half assed or whatever - its what I"m doing!

So what I am asking the body experts is - Do I need to use a sealer over the existing mud and fresh metal to get the new mud to stick to the existing mud?

Should it be seal , new mud , then primer and paint ?

OR

seal , primer , new mud , primer again,paint ?

OR

new mud onto old mud , primer , paint ?

I wanna do it the best I can BUT again going all the way to bare metal is not a route I am willing to go.

Directions,suggestions,flames open to all (almost) ideas.

Thanks,Oldmics

 

I've seen old quality mud sanded and once it was, new mud was added, need to have the tooth so to speak.

Like to hear how the real guys do it...

 

Just take it to Earl Scheib, man

 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I"m too cheap !

Oldmics

 

I grind surface with a 36 grit roloc, blow it off good and reapply filler. No problems yet (touching wood)

Make sure the filler is removed next to any new welds, the intense heat will burn and lift old filler.

 

Stop playing with your wood!

 

You want to feather edge the mud in 36 grit, and 36 grit on the new metal. Don't leave a edge or shelf.
Be sure to remove any old mud that got warm from welding because it's no longer stuck to the base metal. Also, if you knocked the panel around with a hammer there's a good chance that the old mud is loose from that too.

Then just pave the new mug into the 36 grit tooth.
Remember thought, it's only going to stick as well as the loose or disturbed mud. it doesn't take much heat to remove all of the mud so I always take it all off because I don't want to see it again. I'm too cheap to be doing it twice

 

lots of top end restorers here, you need advice from a production body shop guy

 

Make sure the existing filler is well adhered to the metal. You will need to sand it to give some tooth for the new filler. Don't use 36 grit it is way to course for modern fillers you don't need to go any coarser than 80 grit. You should then feather the outer edges of where your filler is going to go with the same grit you are going to finish your filler in. Most guys use 180. Heavy grit sanding scratches are one of the biggest causes of pinholes and the need to use too much primer when they don't get properly sanded before priming and you have to pound on primer to fill them.

Sent from my HTC One_M8 using The H.A.M.B. mobile app

 

See there,
Nobody should use 36 grit because some guys don't know how to use it.
Apparently some body guys don't know how to sand either so all they can use is 180, less damage done by 180 grit. It's hard to argue with

 

I have done what you are asking about....was told many moons ago by a bodyman who started back in the fifties....
what you need to do is....when the old mud is exposed is give it a wipe down with thinners.....when dry...good to go.
plus when finished with the mud if you give it a light wipe down with thinners it will reveal the pinholes

 

No there is just no need to add sanding steps to your process and you certainly don't need to add unnecessary potential problem for a guy who is clearly just learning how to do it. 36 grit is an old technique that is no longer required. I work for a filler company and what I just laid out is the recommended standard for using fillers these days.

 

Grind away loose or broken bondo leaving the stuff that is still stuck on good. Grind the bare metal. You don't need to waste a lot of time, just make sure it is roughened and any loose bondo is gone. Fill and sand as usual.

 

I read on my can on evercoat lite weight use 40-80 grit, so i guess i should go a bit smoother, although i don't usually bite in hard with the 36.

 

i agree with not adding 36 grit scratches if not needed but in this case i would do it. unknown body work of an unknown age, i think i would add the extra sanding step to insure a mechanical bond. i would feather the out side edges of the existing filler with 80 grit and work the edge of the paint back [that can't be removed off the edge of the panel] with 180. epoxy prime to lock it all down after body work is done, use a filler primer, then give it a good long time to dry before block sanding and painting.