Body / Glass: Bondo, Bondo everywhere !!

[mmagic]

My '60 Champ project has a number of places where old Bondo has grown rust underneath and is cracking off or showing tell-tale brown surface cracks. I hadn't planned to take it all to bare metal but it is beginning to look unavoidable as I'm finding rust under paint both under bondo and elsewhere.

I'm using both 25% vinegar washes and Master Series products to kill the rust and seal the surface. I'm impressed with the way their products apply and they are less expensive than Rust Bullet or Eastwood. But, get it on you and you will wear it to Church two Sundays.

The most curious bondo is the step-side fenders where bondo approaches 3/8 to 1/2 inch thick. There doesn't appear to be serious damage unless perhaps a load of rock was dumped on them leaving a lot of minor dings.

Has anyone else found what appears to be excessive bondo on step-side fenders ?? Could this have been factory applied or was this applied at a previous paint job?

[(S)]

The factory did not use 'bondo', ANY bondo without heavy 36-80 G grinder scratches will fail if not more than 3 coats and can be 3/8 ", but is pretty thick in only the worst small areas of a large dent. 1/8 is as thick as you ever want it.

On steel fenders that are waiting to get blasted by rocks, it is a good idea to use metal etch primer, epoxy primer ,a flowing chip guard, color/clear coat, or single stage paint on the backsides.

Clean metal and good primers will protect better than most anything else. Thick or even thin 'rust converters' almost always fail on a car/truck body they are good on inner fenders at seams and frames, small parts not seen.......

[52 Ragtop]

First and foremost, plastic Filler (bondo) is NOT made to be 3/8-1/2 inch thick! It is made for a skim coat AFTER the metal is close to where it is supposed to be! Etch prime, then a good 2 part epoxy primer should be used.
remember, primer is NOT water proof! so, don't prime your work, then put it out in the rain!!

Jim

[mmagic]

Thanks....
A rare 1/8" was my max. I'm having to pull spots that were previously filled to achieve that. I was just shocked with the depth of filler on the step-side fenders when there did not seem to be any reason so I was deducting the question, could it have been a factory treatment?

Yes, I'm using etch primers and 2 part epoxy. And, it is covered/tarped when not being worked on.

S, Are you suggesting against my treatment of surface rust spots? Remove as much as I possible, treat the area With Mastercoat Metal Prep and coat with Mastercoat Sealer/Surfacer ?

[tbirdtbird]

Suggestions for body work

I'd not use vinegar to treat the rusted metal. You mention several commercial metal prep products which I am not familiar with. If these products are phosphoric acid based, then this is what you want. Fozz is one brand I use, and Ace hardware has a product I forget the name, which also has phosphoric acid, and it is green in color in the jug. I trained under a high-end body guy years ago. This is what we did. You mechanically remove as much rust as possible, grinding, needle-scaler, etc, whatever. Then use phosphoric acid (be careful but it is similar to the phosphoric acid in Coca-Cola so don't go crazy). **Even if the surface seems to be free of rust** Meaning all metal is treated this way before applying primer or bondo. If , after grinding, you still have more rust than you would like heat the area gently with a propane torch. (this causes deeper penetration of the acid). Wipe away any excess. (I use it in a spray bottle). The surface will turn a greyish color and almost look like galvanize. What happens is the phosphoric acid reacts with iron oxide (rust...very active chemically) and converts it to iron phosphate (totally inactive chemically). Once you have done this, you have done all you can do. You apply the primer of your choice over this surface, which is the best prep to the metal possible. If you want to go crazy you can apply 2-part acid etch primer over this (all the paint co. have this such as Matrix, Dupont, etc). It actually has a phosphoric acid resin in it and sprays on as a light yellow-green color. But usually I reserve the acid-etch primers for when i need to paint aluminum or galvanized metal, when it is a must to do so. Be sure to follow with a good quality 2-part high build primer and keep it out of the weather until color goes on! (Not just a tarp, but indoors). I have done body work and painted 12 vehicles of my own as far back as 15 yrs and not had a paint or bondo failure using these methods.

If you have areas where you **have** to fill gaps deeper than 1/8th inch or so (and we all do) then use a fiberglass reinforced polyester resin product such as Kitty Hair, Marglass, Fibral, etc. Just be sure to leave the top 1/8th inch for regular bondo, since primers will not stick to glass reinforced products. These glass reinf. products are also much more water resistant than bondo, which absorbs water easily

BTW it is a myth that bondo has to be applied to bare metal. It can be applied over a good quality primer (generally a 2K product or epoxy primer), bare metal (Fozz it first!!!), or scuffed or sanded topcoat, or over itself of course.

[mmagic]

I accept your suggestions as gold.

They are generally consistent with what I'm doing. Even when I've used vinegar and wire brush to cut through rust, I've followed that with MS's phosphoric based prep. With both of these treatments I often use an 80 grit DA or air wirebrush wet. However, regardless how deep I dig, I am always concerned that I could have gotten deeper into the rust. Your point re: use of glass resin products is very welcome as I hit the first such spot on Tuesday and I was not impressed with my outcome using straight bondo. It will be removed and the glass resin used.

Unfortunately, I don't have space to do all my work inside, so I can only minimize exposed time by completing sections such as one side of the bed quickly and moving on section by section. I'm doing Doors, fenders and hood off the vehicle inside. When I get to the Speedster I hope to move it inside.... but the garage is only 18' long and the Speedster is 17' so I anticipate painful shins all winter as I crawl over, under and around it! Curses to DR Horton for out-cheaping themselves on a new house !!

Thanks for your guidance.

[tbirdtbird]

well once you have ground down all the rust you can, even if you are happy with the outcome, you will still have microscopic rust pits which will begin the cancerous process all over again. On those areas you are not happy with, maybe even all areas, spray on the Fozz liberally and get the propane torch out. In some areas I will do more than warm the metal I will get the Fozz boiling (it doesn't take much to do this). This really drives the acid in deep. Then I'll wipe the excess. If I get any rust color on the cloth I repeat the process until no more rust color. The policy in my shop is all bare metal, even newly welded-in patches, get fozzed and at least warmed or heated by torch at the end of the day. I have a chunk of Model A frame rail I treated in this manner 20 yrs ago which has been stored in my basement, and it still has that fresh galvanize look to it. It won't hold up to actual rain, but will resist atmospheric moisture well. If you have storage issues and must keep a vehicle you are working on outside then use epoxy primer as your first coat. It is weather proof. The typical 2K (catalyzed) primers are *not* weatherproof for reasons I can't figure out. You'll sleep well knowing the weather and environment are not undoing all your hard work, despite the added expense!