another everlasting debate (body work)

Terry NJ

Terry NJ

Well-known member
This AM I got one more shock watching You tube videos about filling dents! Years ago, I did some body work and I recall always having bare metal to bond to. Next came a few years ago, It was never to bare metal, always bondo, evercoat etc. over primer! Now, we've come full circle, It's back to filler over bare metal again! I give up! What I'd really like to see is what the professionals do. I stripped, wire wheeled, and lightly sanded my fender till it was bare. I then washed it with Vinegar. Then I pickled it with a phosphoric acid product. Then I used liquid metal to fill in the dents and hammer marks and primed with epoxy primer Some sanding, then I put filler in the low areas, Then a coat of sandable primer. Then a coat of Transtar Black primer. Now I want to put a light coat of filler in any low areas, sand it a little and shoot it with more primer (Filler), then my final sanding. These are kinda rough fenders with a lot of small dents from the underside. I realize that the pros would have gotten them done by now, Is it too many steps? My daughter is getting this car when I go and I want it to last, it's not being flipped.
Terry
 
Brent is an authoritative source to comment. My method is to put filler over catalyzed primer as long as the primer has not been on at least 24 hrs and not more than 7 days.
If more than 7 days, scuff it.
Seems to work for me but not on the cutting edge of all the newer products....old school but try to be compliant with directions!
 
I don't think it really matters. Oldbluoval is right about filler over epoxy primer - scuff it, if in doubt before applying filler. I have done it both ways on the three vehicle I have done and it doesn't seem to matter. People swear doing it both ways, but does it really matter either way?
Rusty Nelson
 
From what the PPG tech told me, it is better to apply filler over cured epoxy primer because the epoxy is a moisture sealer. Applying the filler over bare metal MAY be a problem later if moisture got between the filler and the bare metal. I used to apply over bare metal, and had good results, but have followed the PPG recommendations in the past 20 years. I have seen filler pop off poorly prepared bare metal, or bare metal with some surface rust or rust pits under the filler. The wait time and the scuffing with a scotch pad on epoxy over a week old is important.
 
The science says from multiple sources that it is best to do epoxy first then filler.

The epoxy over properly etched (mechanical or chemical) surface will yield maximum adhesion. Then while the epoxy is 'open' in the recoat window lay down the 2 part filler.

Now let in some reality.

Filler direct to properly etched metal is a pretty darn good bond too.

The reason why shops do bondo first then epoxy is speed. It is a faster process. Hobbyists may be better off epoxy first and my reasoning is that the epoxy will seal the metal as hobbyists often do a little here and a little there.

Now where everyone goes wrong is not following the 'rules' for the coating you are putting on the metal.
First read the tech sheet. There is going to be a certain grit needed for different products and no 40 grit is not usually the correct answer. Sandblasting is a great etch, but when in doubt more scratch per inch is better. So if the tech sheet says 80 to 120 the 120 would be better. I have read the tech sheets for a number of fillers and yes they have different grit ranges depending on the type of filler.

I know epoxy and I believe fillers all need a base PH to cure. If you use an acid product then you risk preventing the layer of material next to the metal from curing. Yes it can fail.

If you do not put the next coat on the epoxy before the end of the recoat window then you must sand it and respray.

When in doubt ignore what your are told by ordinary guys who think they know something and follow what the manufacturer tech sheets or the guy at the manufacturer tells you. As I have learned the hard way, the guy with 30 years of putting paint down may not actually know what he is doing.
 
I need to add. The owner of SPI told me that the best time to apply filler was during the recoat window of the epoxy. The surface of the epoxy has 'holes' in it that close up as it cures. The strands of filler get into these holes and the heat from the filler curing cures the epoxy faster and the filler is tightly attached.

If you are going over cured epoxy you will need to consult the filler you are using tech sheet and sand with the proper grit paper the epoxy.
 
Terry, things have changed, and it is basically because of the cost of labor. We apply filler DTM (direct to metal) simply because we are taking steps to ensure the metal is clean, -AND the metal has been scratch-sanded where there is adhesion for the filler. You must remember that the collector-vehicle market for PPG is very small comparatively speaking, and most reps just do not have enough experience in this area to give prudent advice.

As stated above, use a 24 grit Roloc scuffing pad first over the area you are wanting to fill, then you can be assured it has the ability to bond to the metal.


Now if you really want to scratch your head, imagine if I were to tell you that we have just about phased-out using sandpaper in our shop. We have been transitioning to a new product called AbraNet. We also have experimented with a new primer that is rolled on instead of sprayed. The high-build version is in development now, and we will try it as soon as it is released. As I mentioned above, the technology is changing much because labor is getting too expensive for redundant operations. Kinda like the farmers rarely plow any longer and no-till crop planting is the norm.
 
The technology is changing so fast it is hard to keep up. I just painted my OT motorcycle gas tank and if you have ever tried to block wet sand the primer coat before laying down the color it's a real PITA because there isn't any flat surfaces. Enter the sanding sponge. I kid you not these are the greatest thing since swiss cheese.

They worked perfectly. I actually found them at Menards of all places.
 
flatford39 said:
The technology is changing so fast it is hard to keep up. I just painted my OT motorcycle gas tank and if you have ever tried to block wet sand the primer coat before laying down the color it's a real PITA because there isn't any flat surfaces. Enter the sanding sponge. I kid you not these are the greatest thing since swiss cheese.

They worked perfectly. I actually found them at Menards of all places.
Click to expand...


Actually, there is no need to do that, and in some ways is frowned upon in the industry. Dry sand it with 400 grit and then shoot your topcoat of paint. If you color-sand with 2000 and higher grit paper, then you will need water with a dab of soap to flush the paper.

As far as curves go, the shapes of a M/C tank are similar to shapes of a Model-A front fender. ;)
 
Whew! Between Kevin, NJ Brent Terry and the others, My head is spinning! Roll on primer? Hey that sounds great! Years of flipping houses made me pretty good with a paint roller, but I never thought I'd ever be using one on a carbody! I'm going to disagree with the sponge though. As I said before, the fenders seemed to receive most of their damage from under neath. The fenders have some "Mumps Bumps"! Some of these I was able to tap down to almost flat, anyway, all sanding is done with a block. If I were to use a sponge, it would just follow the highs and lows , the contour of the surface, There are things I want to knock down and I think that I'm getting that done. For sand grit, I started with 80, then 150, then 220. I was planning on then using 320 and finishing with 400. I guess my problem is too many changes, too quickly. Ya can't put this stuff away for a while, then come back to it and it will be the same! Thanks Guys!
Terry
 
I used a shrinking disk and got the metal in the fenders fairly level before I got to paint. Then worked it with a file. I do not have much filler on my fenders cause I wanted all the details in the metal to be visible.

I hear you about too many changes, but the truth is the new stuff can be a time saver. The added plus with a roll on primer (I have not used any yet) is you can work in a residential area without putting out all the fumes. A big plus for neighbor relations. I went to powder coating many things because of the fumes issues.

Maybe we can talk Brent to take a bit of time showing the use of the new sanding fibers and the roll on paint?
 
Kevin in NJ said:
Maybe we can talk Brent to take a bit of time showing the use of the new sanding fibers and the roll on paint?
Click to expand...


Now, so that we are clear, we are not speaking of roll on paint, -but roll on primer. There are several type of this primer which have been out for awhile such as PPG's SX1060 and others, but the technology is ever-evolving and they are now coming out with higher solids primers. Go look at this tech sheet to see the specifics. https://www.tat-co.com/TranstarAuto...ish Datasheets/transtar_tds_6461.pdf?ext=.pdf

When they have the high-build primer/surfacers out, we will look at transitioning to those too.

As far as the sand paper, go to Mirka's website and read about it;

https://www.mirka.com/en-US/us/productview/#/AE0/AUTONET

or their
https://www.mirka.com/en-US/us/productview/#/540/ABRANET


The benefit for us using this is we have a vacuum system in the body shop because our body shop is air conditioned.
.
 
Brent, The Mirka product I was using when I was a lowly Tool and Diemaker in the late 70s and 80s, also the guys who hang sheetrock use a flexible sanding head on a pole to sand off the "dingle berries" and fuzz on their tape and spackle jobs. The sand paper is usually this abrasive screen. The abrasive screen was used in our shop for polishing the parts of the mechanisms we created, but they were always polished against a plate. The actual surface of the screen is very irregular. Whether or not this irregularity transfers into the soft surface of Bondo, I don't know. I have some of the Mirka type product and I will give it a try. I always liked the screen because it doesn't load up like sandpaper.
Terry
 
I make the cherry display tool boxes and as Brent stated hardly use sandpaper as the abranet is so much better. Just saw where Woodcraft has it for belt sanders.

Sewall Tyler
 
My advice is to those who rush into bondo. Go around the car with a straight edge and if you see light u;nder the blade (indicating greater than 1/16") you haven't done enough metal work. Dents are to be worked out not covered over. Get a studwelder/dent puller if necessary but bondo should be no more than a couple of paper sheets in thickness.
 
I remember when is was just starting out in the auto body world, I was using a straight edge on a door and my boss told me if he ever saw me doing that again, I was fired. Most panels have a slight crown to them, so a straight edge might not give you the information that you are looking for.
 
Here is the way i do bondo/body work. And i consider this to be the absolute best way to do it for the longest lifetime of the paint job. First is strip any old paint off by hand, then sand blast. stripping by hand decreases the chances of warping the panel when blasting. Before blasting, i do any metal work that needs to be done, patch panels, metal work, and all welding. then blast it. Once the panel is blasted, i immediately (within an hour) coat it with Etch primer. Etch primer has acid in it that neutralizes micro rust (rust you can't see). It will NOT neutralize dark rust spots or surface rust, it needs to be good clean metal. After 2 coats of Etch, wait 15 minutes, and then seal it. I use a very expensive primer called NP75 from sherwin williams. Any paint rep will tell you it is one of the most badass primers on the market. I had an Dupont rep tell me one time that he wishes they could make a primer as good as it. NP75 is an epoxy/Urethane hybrid that is a sealer and high build in one. It is DTM, which is the epoxy side of things, and has the build and lays out slick like a Urethane. but really any Epoxy sealer would work just fine at this stage also.

Once i seal it with the np75, 2-3 coats, I then go back and SCUFF my bondo areas with 80 grit, being careful to not break through to metal. Don't ever apply bondo without scuffing. Bondo gets its adhesion from the mechanical bond of the sanding scratch. I do my body work, and get things ready for the next round of primer. Before i spray my next round of NP75 for block sanding, I then spray over my bare metal spots again with etch primer. Once you sand through and get the old etch off, the corrosion prevention is gone, so you need to re apply it. Then wait 15 minutes again and apply the NP 75, and do this repeatedly through my multiple blockings, until i have the panel laser strait and ready for paint.

this is the only way i can see doing these paint jobs that will guarantee my customers the best bang for their buck. I don't want to see any of my jobs having rust bubbles 5 years later because i didn't use the best possible methods for rust prevention, and allowing my body work to have a long of a lifetime as possible. My gallon of Etch primer cost me $325, and is worth every penny of it for the insurance. I have done paint jobs without it before, properly stripped, and immediately sealed and within a couple of years it had some minor rust spots coming up, which i took care of for free because i value my customers. But this is just my .02 cents, and i realize i have a biast opinion on this because this is how i make my living. I'm not the average joe at home just trying to do a paint job on a budget.
 
Thanks Logan! I can't see myself spending $325 for a primer, but it's good to know it's out there! I appreciate your input!
Terry
 
Terry said:
Thanks Logan! I can't see myself spending $325 for a primer, but it's good to know it's out there! I appreciate your input!
Terry
Click to expand...

There is cheaper etch primer out there. You can get gallon set ups for 100-150, I just use the top of the line. On any given car that I paint I have about $2500-3000 in paint material. Some people call that crazy for a model a, but the material is the cheapest part of the whole paint job.
 
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