The Stovebolt Site Forums The Shop Area Paint and Body Shop Metal Etch body after Media Blasting ?

Seems to be a fairly controversial topic but looking for the latest thinking. Just started blasting my panels, and want to stop cancer while working on driveline. So after media blasting to bare metal to the point where I cannot SEE rust, it seems that at a microscopic level, there must be some residual rust remaining. Logically, it seems to get it all, a light wiping with a rag (not heavy brushing) of a phosphoric acid based etch, let dry overnight, wipe with a wax/grease remover (DX330 for me and a PPG system) would be best before applying an epoxy primer. MSDS link for PPG primer below.

Doesn't seem to be a consensus, but if you guys tell me you've skipped the etch, and had perfect paint for 10 years, I'll let it go. Appreciate your experiences.

PPG Primer Link

One thing you have to understand, rust is not alive. It is iron oxide. It is caused by a reduction reaction of ferrous metals in the presence of oxygen and moisture.

The key is "in the presence"!

If you keep iron away from moisture in the presence of oxygen it can't rust.

Rust can't grow on it's own.

Even if you have visible rust, covering it with a moisture proof, air proof film (good paint) will keep the whole surface from rusting.

If the film is breached, the clean shiny metal will rust even faster than the metal already coated in rust.

Stories of repaired body work rusting out again is almost always caused by air and moisture coming through from the back of the part.

Allen, I would go with PPG's recommendation as it appears you have their products specified. Call their tech line and get it from the horse's mouth.

This was in the link you posted:


....where the 171 is an etching primer. I'm not that familiar with all the PPG products, perhaps there is a different etch that is compatible.

Let see if this makes any sense Allen. You've sandblasted all
the old paint, primer, fillers and rust off the metal leaving
a rough (etched) surface. So...why would you need to etch it
further with an acid? Etching primer is used on bare metal
to give it tooth, how much more tooth do you need beyond
sand blasting?
Here's a fender & running board that 8 years ago were blasted
with black max then primed with PPG-DPLF. The running board
was painted with Rustoleum the fender is yet to be finish
coated. The truck has been driven all that time as is and
I've put about 10K on it. It's been parked in cold garage
in the winters, and she's been caught in the rain many times.
No signs of rust:
http://www.pbase.com/dennygraham/image/156702149/large

DG

I agree with the others, but if I was really concerned I would use PPG's own metal cleaner and conditioner which are based on phosphoric acid. It's recommended for their epoxy primer so it couldn't hurt.

But when I look at older restorations with failing paint, in all cases the paint jobs are failing due to excessive paint thickness or poor adhesion compounded by excessive paint thickness, not rust.

You are on the right track.

I used the same Phosphoric based potion from PPG as directed AFTER I had the panel media blasted. You'll notice a kind of hazy rainbow effect on the metal surface and that is normal. In my case that was enough to ward off any surface corrosion
till paint time ; while I started sheet metal repairs and chassis upgrades.

proceed with confidence!!

mike

So Allen, sometimes you KNOW the correct answer, and just want confirmation before proceeding. This is one of those times. According to every paint forum I am on, etching primer is the old and less quality way, and epoxy is the new and best way. Even my paint supplier agrees with exactly what you posted in your first post. DPLF is great epoxy primer, but whatever you do, be SURE to observe the 30 minute induction period. It matters!

Great discussion fellas. That's what I was hoping for...so fair point Deve. Denny, thanks for the long run time example. I took Robert's advice and called the tech line today. They recommend blast, apply DX330 cleaner (no rust converter), then epoxy prime right away, particularly down here in a high humidity environment, overnight at the longest. Their concern is that some of the acid that doesn't chemically convert, pools in pores, and reacts with final coats and bubbles up later. I hate rust almost as much as I hate fire ants, both are equally hard to get rid of. Just looking to confirm not creating long term problems by not treating the rust while I have the opportunity and am down to bare metal. Jim's points provide good perspective. darn sure don't want to repeat the blast process. Thanks again!

Ok, but there is another concern that's pretty important. You sandblast, DX330 clean, then epoxy prime. Ok, good job, but in SEVEN days you are out of the window for the next coat!

So if you do that and just blow off the 7 days, you now have to go back and rough up the epoxy primer before applying high build primer later. Since I don't need to tell you how hard it is to rough up every square inch.. do NOT miss one single square inch.. before primering with the next coating, thinking about it just gives me the chills... don't stop there!

Its very easy to just go straight from DPLF to K36 Urethane high build primer (recommended) and THEN put your parts away for later. The reason you do this is because K36 has no window and you can just sand it later as you would anyway and there is no penalty. Epoxy primer is only open for 7 days. If you are like me and hate doing things twice, especially things like sanding.. its something to really think carefully about.

How soon after DPLF can you apply K36? Assume single coat of each. Heck of a lot easier while I've got parts hanging on the parts rack to shoot 'em. It may be 2-3 yrs before I'm ready for body work/final coats.

Yep. exactly. I would wait a day, let them sit overnight before hitting them with K36. I am sure the TDS will tell you the exact numbers. I am at home so don't have the numbers in front of me.

DX330 is a wax and grease remover. If I had a painted panel that
I thought had been waxed heavily or one that was greasy or oily
them before I blasted it I would use a wax and grease remover
to avoid contaminating the sand or leaving residue behind after
the media blasting.

I should have added that after blasting I use a conversion
coating or metal conditioner like PPG DX520. I use a DuPont
cleaner and conditioner that I've had for ages cuz years ago the
paint line that I used mostly was DuPont.
I had the front clip dipped at a chemical stripper back in '09
and the last step after a good rinsing is to spray them with a
conditioner. The parts hung bare from the ceiling in my garage
for a couple of years before I finally got around to working
them. No rust on them which I credit to the conversion coating
which the stripper applied.
http://www.pbase.com/dennygraham/image/138319368/large
No problems with the paint either, which is about
five years old now.
http://www.pbase.com/dennygraham/image/139199169/large

The phosphoric acid based conditioners on bare steel, and
galvanized leaves a insoluble, crystalline phosphate layer on
the surface which makes for a good corrosion resistant base
for the next layer, be it paint or primer.

Denny G
Sandwich, IL

And don't use etching primer over an already etched surface.
Just plain epoxy okay.

Before we put this one to bed, I called the tech line again after seeing on the DPLF tech sheet that it can be applied over a "converter" and seeing Denny's post. I have had very good luck over the years with the same formula, knock all loose rust down, hit it with ospho, then prime and top coat....but this is for mower decks, tractor implements, etc...never an auto body. So natural for me to apply a rust converter after blasting.

In the end, PPG recommends 2 options 1) Option A - blast, clean with DX330, then use DX579 & 520 which are cleaner and conditioner (as Denny says above). This requires a water rinse. Then apply epoxy primer,etc. If I was going for the Ridler, maybe I'd sign up for this.

Option B - blast, clean with DX330, then 2 coats of epoxy primer just as soon as you can, preferably that day, next day at the latest in my humid Houston,Tx climate. On a scale of 1-10, if option A is a 10, Option B is an 8, according to tech line.

Further, in terms of long term preservation, which is my situation, maybe a couple years. (Don't apply K36 Urethane primer yet Deve !). Suggestion is when ready to start body work, scuff the old epoxy primer, apply filler, then another coat of epoxy primer (a sandwich around the filler). Then the K36, then base coat/color and clear if you're doing base/clear. PPG says you only need a sealer after the K36 (urethane primer) if you're painting black. Black pigments, need a sealer for adhesion. Thanks again fellas for good discussion.

In your climate Allen, I'd for sure use a conditioner before I
primed it. It will help down the road in preventing rust, also
in the adhesion, but with the tooth you have from sand blasting
I wouldn't worry about adhesion.
It's really not a big deal to apply the conditioner, just swab
it on, let it sit for a while and rinse it of with the garden
hose, and away you go.

DG

Man! Just when I think that I am confident in how to proceed with prep and paint, somebody throws out something else to think about. Now I must downgrade myself from just barely knowing squat about painting, back to not knowing squat. I may just buy a couple gallons of barn paint and a big brush. Later touch up will be a breeze!
Carl

Come on Carl, it's not really all that critical. Climate wise
I'd say the here in the mid-west we have a balance between the
east and the west, wet at times, dry as a bone at others. I've
blasted bare lots of metal and simply sprayed it with Rustoleum
rattle can enamel paint with no conditioners or even primers.
Had a trailer frame that sat out back of the barn for 20 years
and it was still solid, restored lawn mowers & tractors painted
the same way, stored in the shed and they've lasted even longer
than the trailer without rusting up.
Blast, swab & rinse with some conditioner, prime with DPLF
primer and paint, that's not all that complicated. That's all
I've done with this truck, ain't no show winner but
for a driver it's plenty good and will probably outlast me.
http://www.pbase.com/dennygraham/image/139199169/large
Now if your looking for a class winner super show worthy paint
job then you'll probably consider all the rest of the stuff,
like cleaners, conditioners, epoxy primer, sanding sealers,
base coat clear coat.

DG

I think a lot of this is about long term storage of unpainted items and how to keep them fresh. In talking with my PPG representatives here, after cleaning the sandblasting dust off with DX330, do it right, and do it thoroughly, DPLF and K36. They KNOW what the PPG hotline says.. but they say do NOTHING that requires a WATER rinse. Why put water? on your metal when you are worried about your humidity? They also say K36 years later will be fine for applying filler, etc. Its all arguable though. You will do what you feel is best and it will all work out.

On sealer.. experts agree it isn't necessary, its just prudent. If you stop at 400 grit paper before finish painting, the sealer fills 320 grit and above scratches completely. This is just cheap insurance that your scratches aren't going to come back to bite you. The added benefit of it being epoxy sealer so you are effectively sandwiching in the process.

Any of the suggestions in this thread will make for a 'Riddler'. But what makes this possible is knowing what you are doing in the application process. This is what scares the crap out of me.. every time I step into the paint room! Will I be good enough in my technique to not run the paint, or whatever on that particular day? I certainly hope so.

I guess you are right Denny, I need to quit laboring over all of the what ifs and just have the dang thing painted by someone who does it for a living. Thank you for talking me off the ledge.
Carl

this was my horrible experience....I had a friend blast my parts and my nearly rust free cab...it went from that buddys house straight to anothers who was gonna shoot it with dp40....this friend insisted that we metal prep it first...I was totally ignorant and followed his instruction.....I was horrified as I literally watched my freshly blasted perfectly clean cab rust before my eyes...it was God awful....it was such a pain in the butt to get it to where we could finally spray it.......dave

I know right? You metal 'prepped' it when you sandblasted it. Its down to bare metal, all you have to do now is clean the dust and possible oil contamination from touching or the oils in the compressed air used for blasting. DX330.

I had a friend that insisted the proper finish sanding before base/clear was 600 grit WET. I actually had to replace some of the areas he did with that IDEA with new metal! Come to find out some automotive restoration programs are actually teaching 600 wet before base/clear. Wow! The graduate who told me that and now owns his own restoration business agrees that was the worst instruction he ever got. 400 grit dry, sealer and done!

Whats sad to me is that experts from all over the world in this profession do not get together and post actual working instruction for everyone to follow. Maybe in 30 years or so. This would eliminate all of this conjecture. With nobody on the same page, we just do what works for us. Its just a truck right?

that summed it up perfect...."we just do what works for us".....dave

The procedure you use depends on the climate where you are doing the work. In the Houston area, you need to be ready to spray the EP on as soon as you get finished with the blasting. You will save yourself a lot of work.