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Bondo over Primer

Bondo is a wonderful filler, but! only to be used UNDER any type of PAINT or other body filler, it needs something to hang on to, either use a 16 open coat grinding disk, or drill 1/16th holes where it is to be applyed, it will stay forever... main thing is this.. do not apply too think at a time, like more than 1/4 inch at a time, let dry fully between coats, a slite 16 grind when totally dry, at times a few days, if it is not hot, then apply your second coat, same for more... Green stuff body filler is nothing more than thick primer, to be used over primer... and yes the guide coat process is the only way to see where you are at...
You CAN put filler on bare metal or glass with an 80grit scratch...thats PLENTY of adhesion for the bondo. Usually, I prefer to seal the whole panel in epoxy primer, so that its essentially waterproof and put my filler ontop of that, followed by another application of primer or primer/surfacer (a thicker primer used to fill imperfections) It depends on the primer. Some primers cannot be applied over filler, they'll soften it. Some can't have filler ontop of it either...same reason.
PERSONALLY I use epoxy primer... Its waterproof, sticks pretty good. Then you scuff sand it, apply my bondo and then you SHOULD use a primer/surfacer...and blocksand that. What I did was paint my epoxy on a pretty rough scratch, let it cure for 3 days, shoot a guide coat, blocksand and fill any imperfections with Rage Gold or evercoat. Followed by more epoxy and a topcoat. Thats what Id do anyway. If its really wavy after, shoot some primer surfacer and keep blocksanding.
On metal, I skip etching primer all together, because usually they suggest a sealer on top of it, and its a lacquer based substrate(not worth my time). Drilling holes in panels and squeegeeing bondo in it, kinda works...IF you keep it thin, BUYT any moisture will get behind it and negate all your work when it expands and pops half your door off...with the paint. You really wanna hit a mud puddle and worry about half a can of bondo falling out of your car 2 weeks later...or 2 years or 10yrs? Me neither. Yeah its worked for people I dont wanna worry about losing body work on the highway. An 80grit on a DA is all you need for beginning your filler work. Primer be sanded with 320 or 400grit for multiple applications of primer (check your tech sheet with your primer from the paint store - theyre all available online for free from the websites too). Wetsand your final primer coat with 400grit; then tack rag, then wipe with wax and grease remover and shoot some paint on it. Check I learned a LOT there from the pros...good advice from the people who make a living, really ratcheted my work up 100%
You should really use a 2 part primer and catalyzed paint system these days.
Single Stage Urethane or Base/Clear
I recently had this same discussion with a 3M rep. His answer was that the "cheaper" fillers adhear by mechanical means ie: 40 to 80 grit applied to bear metal to make a rough enough surface to cling to where as the better ( more expensive) fillers use a bonding affect ( like glue ) to adhere to the surface so they can be applied over a good epoxy primer.
I was talking Bondo, maybe the first to hit the market, but it was and still is as far as I am concerned Fantastic, I have used it to do extreem fix it jobs, and custom body about a test, try filling and peaking in a 50 Ford hood, then see how long it lasts through all the hood slammings.. Years worth and no cracks, and no water bleed from the rear.. If you can get at the back side while applying the first coat of bondo, make a rivit head on the back side, then when dry you can spray some type of sealer if you wish... I will still go with a 16 open coat grinding disk for bare metal, just don't rub your bare fingers over that surface, as it may cut your claws... I was tought this from guys that were old custom car builders from the old days, I love the old tried and true practices myself... Thank You..
Ted Brown said:
... Green stuff body filler is nothing more than thick primer, to be used over primer...

I used the term Bondo in a generic sense... I actually used Evercoat Xtreme premium lightweight body filler


then on the final coat I used USC ICING...

T-Bucket Bob said:
Hey Guys,

Can you bondo over primer or should the primer be sanded?

Thanks, T-Bucket Bob :lol:

Getting back to the original question - Yes, you CAN put body filler over primer,.. but it's not recommended. Body filler, regardless of brand name, is best and most stable if applied onto bare metal or bare fiberglass that has been prepared by roughing up with 24 or 36 grit disc on a grinder or dual action sander.

The main filler ingredient in polyester body filler is Talc. Yes, it's the same material that's in baby powder and it absorbs moisture if not completely sealed from the atmosphere. Therefore, drilling holes in a panel allows moisture to intrude from the back side and will eventually destroy the filler. If there are perforations in a panel (drilled, rust holes, etc.), the first application of filler should be one of the waterproof types like Dynaglass or Kitty Hair. They have pulverized glass fibers instead of Talc as a filler agent and do not absorb water.
No, that is what Green stuff or any body surfacer is used for. A Bondo type of filler, is used for thicker applications, shaping and the like, all the rest is more like primer, the trick for all fillers is to be completely DRY before any type of paint or even primer is put over it, as then it may never dry completely and it then will draw moisture, when it is completely DRY, it will not... That is why it takes soo long to get a small repair done, no real time to fix, it is all drying time, so anyone that can fix and paint a repair job in a few days, is not doing the job correctly.. Just the way it is... Junior used to charge $50.000 for a paint job, because it took 1000 hours to do the job correctly, and that is only $50 an hour, no money at all in todays standard, so no more super fine paint jobs.. He said that most of the time was drying times... I also have a friend that used to do custom painting on custom Bikes, He would not paint over someone elses Bondo type shaping or filling, He always striped everything back to bare metal and started over from scratch, most people never knew He did this, but He could not Guarantee his work, any other way.
So does that mean, if I want to smooth out corners and where brackets are, I will have to strip the primer off and then apply the bondo?

T-Bucket Bob :welcome:
LumenAl said:
Meangreen... question... I don't have the skills to nail the bondo in one take, are you suggestion that anywhere I need to float or skim coat an area that I take it down to the glass or metal every time?

No, just on the initial application. Normally, body filler is shaped with a Sureform Tool (cheese grater file) followed by 36 or 40 grit sandpaper and then finish contoured with 80 grit. If you find low spots that need additional filler, you can add it directly on top of what you have done up to this point. Skim coats will stick to 80 grit scratches just fine. Where the trouble arises is when you try to put new body filler over old stuff that has been there for a couple of years. New filler will sometimes soften and lift or loosen the old stuff and the edges will not "feather" properly which shows through the finish.

Once you have reached the point of applying primer, minor imperfections can be taken care of with glazing putty. There is the "Green stuff" that Ted mentioned and some others like Nitrostan (red oxide) and others which are lacquer-based putty. They do need substantial dry time or they can cause problems later. There are also epoxy based glazing putties like Icing and Evercoat which require mixing with a hardener. These harden chemically in a few minutes and require no extended dry time, but are more expensive.

For small areas like smoothing weld joints or filleting a corner where a bracket is attached, it is OK to put body filler over epoxy primer. Just scuff the area lightly with some 80 grit and wipe the filler in with your finger or the corner of a plastic spreader that has been trimmed to the radius you desire. After it hardens, sand and prime normally.
Ted Brown said:
...the trick for all fillers is to be completely DRY before any type of paint or even primer is put over it, as then it may never dry completely and it then will draw moisture, when it is completely DRY, it will not...

You are correct that all fillers must be completely dry or cured before the application of primers or paints. Otherwise, they may cause lifting of the edges, shrinkage, revelation of sanding scratches, and/or discoloration of the paint. However, polyester body filler does not "dry"; it hardens via a chemical reaction that is induced by the addition of a catalyst. Body filler contains Talc, even the lightweight types that have the tiny glass spheres added still have Talc. Unless the filler is completely sealed from the atmosphere on all sides, the Talc will absorb moisture regardless of how "dry" or hard the filler gets. The moisture then attacks the metal that the filler is adhered to. It's a slow process, but eventually the moisture causes the base metal to begin rusting and the rust pushes the body filler off.

If you use fiberglass fillers (Dynaglass; Kitty Hair) intead of Talc based fillers to cover rust holes or other perforations, it does not absorb moisture and eliminates the "rust & release" problem. You should only use enough to plug the holes and then finish it with regular filler. Someone may ask, "Why not just use the fiberglass stuff all the time?" (1) It's more expensive and (2) it doesn't finish as well. It has a tendency to have little pieces of fiberglass "pop" through the surface and show in the finish.
When I first bought my 72 camaro, I drove it for awhile without doing much to it (points, condenser, plugs wires and cap). I came home from work and it looked like someone took a softball bat and speared my car right in front of the rear wheelwell. I had gone over the car with a magnet...guess I missed a spot. Driving to work that day, I saw a baseball sized chunk of green "stuff" on the side of the road...mmhmm. Buyer Beware. Took me an afternoon to cut the metal out and replace it, no big deal...but the point is PLASTIC FILLER IS NO SUBSTITUTE FOR PROPER BODYWORK. argh!

On a sidenote, Ive heard stories of lacquer based glazing putty "lifting" when 2K (2 part non lacquer) primer goes on. Yeah the glazing putty with hardener (2K K for Komponen -german for component) is a bit more expensive, BUT its not filler, keep it above 50 degrees and it'll last a long time in the tube. spot repair on your show n shine T is a lot more expensive than a tube of glazing putty. Mean Green thanks for explaining some of the differences.
Note: for those of you that may extend your fiberglass work to urethane bumper covers and or newer vette parts; FIBERGLASS and garden variety BONDO wont stick. Early 70s Vettes to now are SMC (sheet molded compound) which is like fiberglass mat, impregnated with a urethane resin in a vacuum press...It never 'kicks'. That means it never cures. You have to use SMC and Urethane specific bodyfillers and mat/resins on these parts. The nice thing is SMC repair products will stick to fiberglass, but fiberglass wont stick to SMC.
Re: Bondo over Primer(Stipping Off The Primer)

Is there a good product to take off primer? Then, after the primer is off, what can I use to protect the bare metal from rusting or moisture? I won't be able to prime until spring comes around here , which would be about March or April.

Thanks, T-Bucket Bob :D
If youre worried about rust...I would just spraybomb whatever it is youre working on for now and strip it later. Use an enamel spray can if you can, these new lacquers dont dry worth a sh!t with the health nazi's hanging around. metal temp is key...point a salamander or torpedo heater at it for awhile, just till its warm to the touch, kill the heat and spray it. Other option is to strip your primer with say a "roloc" disc (kinda like a scotchbrite pad on an arbor for electric drills. and then wipe it with some WD40 every few days.

If you dont have to do any bondo work on those areas, Id pick out your epoxy primer of choice, see what they require for mechanical adhesion and strip the parts with THAT, then apply your light machine oil, that way when you CAN paint, the majority of the prep is done. or .net sells some good epoxy primer, its cheaper than NAPA but you have to deal with shipping. Once it cures for 3 days or so, you can wet blocksand it and apply more primer and spotputty (should use a primer surfacer for this but you dont have to) then color coat
T-Bucket Bob said:
So does that mean, if I want to smooth out corners and where brackets are, I will have to strip the primer off and then apply the bondo?

T-Bucket Bob :D

Strip off primer? Well if your area is not needing extreem filling, like complete custom frame molding over bad welds... but over nice smooth welds, you can just use a glazing type putty of your choice, applyed over the primer with your finger, easiest... Clean your hands with lac. thinner when finished. As far as bare metal protection, you can always use Metal prep to wash down the whole area that is bare metal,,, WD40 seems to not do real good at stopping rust, (I have found) and I never like to apply any oil where ever I intend to do any filling or painting later, too hard to get completely off... But that is an option, just clean very well before any type of metal work starts.

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