Can you put body filler over primer?

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Annika

Annika

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Hey guys...

Popping in to ask a few questions even though I've been off the planet for months. :oops:


The bike is coming along and I'm excited to get it done. But I've hit a little disappointment...When I put the body filler on the bare metal fenders and rear rack of the bike, and sanded it down, I thought it felt and looked great. Applied black matte etch primer, still thought it looked great. Today I applied acrylic primer to prepare it for topcoat....and lo and behold, LUMPS! It turns out I just did a mediocre job with the body filler (it was my first time).

My question is, what can I do now? Can I sand it back a bit, apply more body filler, and continue on? Do I really have to go back to bare metal? I'm hoping I can 80-grit sand and apply more filler....but I DON'T want to make a small problem big.
:?:
 
In my experience, it's better just to take it all the way back down to bare metal and start fresh. That way you know its going to adhere, and also, you won't be putting any more bondo than you need on the bike. If you do just put more down over what's already there, chances are you'll still end up with lumps or uneven spots that will show up when its painted.
 
I have my best luck hiding a transition from filler to metal/wood/whatever ("feathering")with the type of orbital sander w/a rotating head. By hand takes skill and practice that I lack, and plain vibrators don't seem to do the job.
 
Sounds like one of a couple of things. The bare metal wasn't prepped right. I usually wipe it down with a cleaner, like lacquer thinner or even alcohol, to make sure there's no oil or any type of residue on the metal. I also make sure I have the area sanded with nothing higher then 80 grit, gives the bondo something to "grab" on to...The other thing, maybe you didn't MIX the hardener with the bondo completely, this can cause it to bubble in the final sanding stages.............Strip it, start over. :wink:
 
Also, they have glazing compounds that are specifically for touching up imperfections that show up in the primer sanding stage. They're somewhat thinner and easier to use than regular fillers, though not as strong. They're not for filling anything much bigger than a scratch or a sanding line, however.
 
could you post a picture, sounds like you just might be talking about lumps from not being sanded and blended in enough rather than a bonding problem, and some fillers can be applied over some primers, need to read directions on what you are using.
 
KZ1000 said:
could you post a picture, sounds like you just might be talking about lumps from not being sanded and blended in enough rather than a bonding problem, and some fillers can be applied over some primers, need to read directions on what you are using.
Click to expand...

Yes. Totally what he said. :) It's not an adhesion issue or a chemical issue. It's a skill issue. :) I didn't sand the bondo down and curve it perfectly around the fender. You can see flat areas where I sanded too much, raised areas where I sanded too little.

I think what may work for me is to sand the lumps I see, and re-prime. Sand, re-prime. Sand, re-prime. I do not think I actually need to add more Bondo, now that I think on it more. It's LESS Bondo that I need, not more. :)

But in the event I did see some areas that could use a Bondo touch-up, I was curious as to whether I could apply it over the acrylic primer that I used (DuPont Quick Prime). I may not need to at all...just gathering opinions so I don't royally mess anything up!

Thanks guys.
 
deorman said:
Also, they have glazing compounds that are specifically for touching up imperfections that show up in the primer sanding stage. They're somewhat thinner and easier to use than regular fillers, though not as strong. They're not for filling anything much bigger than a scratch or a sanding line, however.
Click to expand...

That sounds pretty close to what I'd need....there are just a few lines and pits the size of pinheads that are bothering me. The lumps, I can take care of those by sanding back and re-priming.

By any chance do you have a glazing compound brand in mind? Otherwise I suppose I'd just google around till I found one that could go over the type of primer I used.
 
sensor said:
use guide coat when you block and make sure you actually use a block not just your hand(and the bondo brand blocks will do more damage than good)....
evercoat glaze works really well....and for pinholes blow everything off really well and get a razor blade to spread the glaze over them
Click to expand...

What's guide coat?
 
Ok, thanks.

I think I can improve these things with minimal heartbreak, a lot of elbow grease, and a little glazing compound. ;)
 
I use 3M Acryl-Green Spot Putty. It works over primer for pinholes but anything bigger I haven't had much luck with. Gary
 
I take it the filler is on a fender?

I only hand sand curved surfaces, and I use a piece of self stick 80 grit 8" diameter folded in half. After smoothing off VW's for years, you can block all you want, but I always had the best results by sanding curved surfaces by hand.

Definetly use a guide coat, after time you will also develop a "feel" make sure you sand a little, then run your hand over it, sand some more, repeat.

One other thing... Ive seen rookies try to use too fine a grit when sanding bondo, which will leave lumps and uneven surfaces... Stick with 80 grit till its real close, then switch up to the finer stuff.

I know some people swear by the glazing putty, But I never use it... After getting the area sanded, blow it off REAL good and inspect for pinholes. If it does have pinholes, I mix up a small batch of the same bondo and fill them with a skim coat, just enough to fill the pinholes and scrape the rest off with the spreader, a light sanding and you should be ready for primer.
 
Thanks RatFink1962 - Yup, it's the fenders, and the rack. All those nooks and crannies on the rack need more careful attention than I gave them, and the fenders will definitely improved with a little more patient sanding.

As you predicted, I used 120 to sand the Bondo on the fenders. I'm a newb. I think as a result, although it's "smooth," it's not shaped just right. So it's....smooth but irregular. :roll:

I'm going to pick up some 80 grit, I have a block should I decide to use it, and some more filler. Part of my problem was, I got impatient during the body-filling process because I was using filler that had dried out slightly. It was curing real fast on me so I rushed a little bit.

Here's my last question (for now)....what I did was, Bondo, then etch primer, (because some metal was still exposed), and then acrylic primer. So let's say I sand my odd lumps down...now I have Bondo and a little etch primer exposed. Do you think I should spray etch primer all over the whole fender again, even OVER areas that still have acrylic primer on them? Should I "start over" and sand the WHOLE fender down to expose all the Bondo again?
 
Sensor... not trying to step on your toes, Ive got 15yrs in myself, doing everything from heavy frame work to painter, and in that 15 years I beat on thousands of cars, working at places from a VW dealership to independent shops, but I switched occupations 20 years ago. so Im pretty old school and I know there is a lot of the newer methods that I dont know about.

I just remember trying to work hoods on VW's using a block and I always had to do it over and over... till one of the oldtimers told me the folded 8" paper trick and sand it by hand, then I was able to knock some stuff out! to each his own though... Im not knocking your method, since it obviously works for you.

So at this point it dont matter if he blocks or hand sands, since the main problem I believe is that he is using 120 grit. It always works better to rough it out with 80 grit then lightly hit it with the finer stuff once its close. But if you have to re-apply bondo, at least use the 80 so it will adhere good.

And not to open another can of worms... Im assuming its a dented fender you are working on? Did you hammer and dolly the dent out? I know since Ive been out of production bodywork and dont fel the pressure of making flag hours, I spend a lot more time trying to metal finish stuff and reduce the bondo. Pick and file works pretty good, takes time though. And if you try it, just remember you dont want to file off too much metal, you just want to re-shape it and use the file to show the high and low spots.

To answer your primer question, I would say if you sand to bare metal, give it another coat of the etch primer.
 
Not a HE........But a SHE!
 

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