Old skool bondo?
 

Alrighty - question for the auto body experts out there...

Dealing with a couple self inflicted wounds (sorry yada!), it seemed to make sense to also deal with the little rust bubble along the right rear wheel well. Peeling back the layers, it is now clear that the whole right side of me car has been "dealt with" at some point in the past.

On to the question: the filler used at some point in the past to pretty much re-sculpt the wheel arch on my car is yellow-ish in color. Smells absolutely NOT bondo-like when sanded. Seems to be pretty dang soft, pulling and shredding vs. nice clean sanding.

Any clue what the H E double hockey sticks I'm dealing with?

probably not mixed correctly and never quite setup.

Have you checked the rear diff for Bondo?

i believe that was sawdust!

I was thinking that someone might have gone to Bondo up the body and banana the diff and gotten confused.

Bondo is not waterproof. If their were previous holes short haired fibreglass should have been the first waterproof fill layer.

It has been my practice to actually build up pretty far with waterproof fillers and then a small layer of bondo for easier finishing where needed. No holes you can use all bondo I imagine.

You have to keep the moisture out if there are any indications of perforation.

You asked for experts opinions. Unfortunatly I am not one. Instead just having some experience with my own vehicles over the years. Plus one of my friends now deceased was an abnormally greatly skilled auto body guy. He always passed along advice when requested.

Even worse in my opinion is if the bondo absorbs moisture from the backside holes or whatever over time it will swell a little. Perhaps not enough for the average guy to pick up but I do sometimes. The paint protects the outside of the bondo from moisture absorption. It is up to the individual to protect the backside where perforations may exist. Short or long haired fiberglass filler does this well and personally I find it more stable as well.

There was/is some gold colored Bondo- I forgot what it is called but IIRC it's their premium line. Still absorbs water though so get it out of there.