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Mollard 07-21-2004 04:13 PM

Bare Metal
OK, so I'm doing some body mods on my Astro, amd I have some places that are down to the bare metal. They need a bit of filler so what should I do. I don't want to put the filler directly over the metal becasue I'm afraid of rust comeing through it (it sat for a few days, so I know theres a bit of rust in there).

So what should I put over the bare metal before applying the filler.

Thanks guys.

Centerline 07-21-2004 04:23 PM

Use a good quality epoxy primer over the bare metal. Then do your filler followed by a quality primer and you should be ready for paint.


Mollard 07-21-2004 04:40 PM

Two Questions;

1) Epoxy primer? Is that a primer with a hardner?
2) For the primer over the filler, would I use epoxy primer again?

mean_1 07-21-2004 05:39 PM

any body filler has to go over bare metal...then if u seal it you have no problem with rust

Centerline 07-21-2004 06:04 PM

I'm sorry but I have to disagree. Most pros, when doing bodywork after an area has been stripped will recommend using a good epoxy (two part) primer to seal the surface first. Body filler is porous and will attract moisture which, if not sealed out eventually promote rust. Not that it won't work without the primer but to ensure good results, especially if the bare metal has been sitting for even a few days, it's a good idea to seal it first.

After the body work you should use a good sandable primer, not epoxy, it's too hard.


BarryK 07-21-2004 06:38 PM

Centerline is 1000% right!

Mollard 07-21-2004 07:25 PM

OK, so I need to know the difference between epoxy and a "good primer". Epoxy primer is two parts, The primer and a catalyst (hardner), right?

What primer would I use on top or the filler and under the paint?

Also, is "self etching primer" and "sealer" just other names for epoxy primer.

troy-curt 07-21-2004 07:37 PM

I have used thousands of gallons of body filler, and never put epoxy under it. Just be sure the metal is clean.

Knock on wood. I've never had rust problems under filler, or had any come off.


If you don't make mistakes. your not doing anything.

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Mollard 07-21-2004 07:49 PM

[QUOTE]Originally posted by troy-curt
[B]I have used thousands of gallons of body filler, and never put epoxy under it. Just be sure the metal is clean.

Knock on wood. I've never had rust problems under filler, or had any come off.

OK, but there are a few pits in the spots and I can see a tiny bit of rust in there, and I can't sand it out. So what would you recommend?

troy-curt 07-21-2004 10:15 PM

Grind it out or spot blast it. I don't know of anything that will kill rust enough to paint over it on the outer body panels. Once the rust is out, use a coat of icing to level it, then prime it and sand level.


MARTINSR 07-22-2004 08:05 AM

I'm with Troy, I have never applied filler over epoxy primer in 25 years of doing this stuff for a living. Now, I must admit, it makes sense to apply epoxy first, it really does. On my next restoration project I may just do it. But I never had and I have NEVER, EVER had a failure (ok, that I know of) with applying filler over bare metal. It is after all THE way recommended in EVERY tech sheet I have ever seen for plastic filler.

Understand, I agree with the epoxy over bare metal idea. Many EPOXY manufactures recommend applying plastic filler over the epoxy. HOWEVER, if anyone can come up with a PLASTIC FILLER manufacture who has in the tech sheets to apply thier product over epoxy please post it here.

All I am saying is applying plastic filler over bare metal "may not" be the best way, but it is the only way according to the manufacture of the product and that can't be "bad".

As far as "most pros" doing it over epoxy. Honestly, I have been in hundreds if not thousands of shops and I can not remember a single one of them doing it, even restoration shops. I am in sunny California, maybe that makes a difference. But I don't remember seeing a single one.

m1e9r5c6 07-22-2004 08:26 AM


Originally posted by Mollard
OK, so I need to know the difference between epoxy and a "good primer". Epoxy primer is two parts, The primer and a catalyst (hardner), right?

What primer would I use on top or the filler and under the paint?

Also, is "self etching primer" and "sealer" just other names for epoxy primer.

I am also new at this and some of these terms escape me. What is etching primer, when do you use it and what is the sequence you apply the layers in.

Would I be wrong in assuming the sequence would be:
1)etching primer if there is slight surface rust?
2)epoxy primer
3)body filler
4)sandable primer

I've asked this before but never get a straight answer, because everyone's got their own opinion. Yay or Nay on the above sequence?

MARTINSR 07-22-2004 08:37 AM

That looks fine. The etch under the epoxy is not nessisarily needed, it is "added corrosion resistance". Epoxy has good corrosion resistance, much better than urethane primer. However, not as good as etch. So if you apply the etch before the epoxy you get the best of both.

Here are some basic explainations of these products.

“Basics of Basics” Paint technology
By Brian Martin

There are many different types (or more correctly, technologies) of products you can use in the restoration or repair of your vehicle. Some have a variety of uses while others are very limited with only a few of specific uses. Proper choice of products can help you get the job done faster and/or help with the longevity of the repair.
Let’s start with some basic definitions. I couldn’t possibly know every paint manufactures terminology or product use. These are generalities and should be used as a guide only to then read the tech sheets of the products you have chosen for proper use. These tech sheets can be found at the jobber and are given away free. Or most manufactures have them on line, USE THEM. They are a wealth of information and can save you many headaches. You don’t need to read every word in the mind numbing text, they usually have a “product at a glance” or something like that will cut to the chase and give you what you need.

Basic terminology’s;

“Solvent” is a generic term and refers to any “reducer”, “thinner” that is used to reduce the viscosity (“thickness”) of a product to aid in spraying or applying. It could be acetone, lacquer thinner, urethane reducer, a special “basecoat” reducer, water, alcohol, etc. These solvents ARE NOT INTERCHANGEABLE; each product MUST be used with the specific solvent recommended by the manufacture.
“Etch primer” an acid containing primer.
“Primer” a product that can be applied to bare metal
“Surfacer” (or “primer surfacer”) A primer that has “body” or solids and is used to fill imperfections and provide a film thickness to sand or block a surface to a smooth base for paint.
“Sealer” a non-sanding product that is applied prior to painting.
“Primer-sealer” A sealer that can be applied over bare metal and then top coated without sanding.
“Flash time” the time you allow the solvents to evaporate out of the film you have applied.

Basic technologies;
“Single component” or RTS (Ready To Spray). This is a product that uses no additional components. Just pour it from the can into your gun and shoot. Examples are: Some plastic adhesion promoters and primers and even some top coats like vinyl colors.

“1K” This is a product that uses no hardener, catalyst, activator, etc. It may have an added solvent, but no hardener or activating reducer. 1K products like RTS dry with the evaporation of solvents and are soluble, meaning that they are could be wiped off with a rag soaked with lacquer thinner. They could in THEORY be scraped off and put in a can with solvent and stirred back to a sprayable condition. Of course ALL RTS products are 1K. Examples: All lacquer products, some synthetic enamel products, and some acrylic enamel products. Because of the low VOC regulations the 1K product options are getting scarce, with most limited to “specialty products” like adhesion promoters.

“2K” or “Two component” is any product that uses a hardener, activator, catalyst, etc. It may or may not use a third component in the form of a solvent. 2K products don’t “dry” like a 1K. The 2K product “cures” by molecules linking together to form a whole new compound. Most high quality 2Ks are insoluble after a full cure and will not soften when exposed to solvents like thinners or gas. Examples are urethane under coats and top coats. Epoxies, ISO free products that use a hardener, etc.

Basic tip, ALL 2K products should be mixed as accurately as possible. As a rule 2K products need a minimum of 55 degrees to cure with an ideal minimum of 65 degrees. MIX THEM AS DESCRIBED BY THE MANUFACTURE. They have spent hundreds of thousands, possibly millions of dollars developing the product, they WANT it to work as BEST it can. Do as they say, don’t become a “Junior Chemist”.

Types of products and their uses;

Etch primers (some are 2K)

“Wash” or “Vinyl wash” are for bare metal applications for the ultimate in adhesion and corrosion protection. They are very low in solids with next to zero filling qualities. Some are even semi transparent. They are usually not to be top coated with paint. You apply them to aid in adhesion and corrosion protection under other undercoats such as epoxy or urethane primers.
- Very thin, keeps down film build
- Cost effective
- Fast application
- Non-sanding
- Super high corrosion protection.

- Some have a very small re-coat window

“Etch primer” (some are 2K)

Typical “etch primers” have much more solids and body than “wash” primers. They are more forgiving than “wash” primers, one thing being a much longer re-coat window. They are basically used to aid in adhesion and corrosion protection as with “wash” primer. You would choose “typical” etch over “wash” if you have some paint or plastic filler as a substrate along with the bare metal. Some brands have a recommendation to apply top coats over it also. This could be very useful in a money saving or time saving is important.

- Easy to apply, smooth, easy to sand
- Some can be applied over plastic filler (not that you need it over the plastic filler, but if you have some, it is nice to not have to go around it)
- Some can be top coated, which can be a big time and money saver.
- VERY cost effective

- Added product to buy and apply.

IMPORTANT basic! If you have used ANY metal treatment or “conditioner” read tech sheets carefully for compatibility . The acid in the metal “treatment” or “conditioner” can attack the acid in etch primers and it can LOOSE adhesion from the metal!

Urethane primer (2K) Urethane primer is the most common primer used in auto body and restoration by far. It has good solids and fills well. It is easy to sand and can provide you with a perfect body when blocked properly. Care should be taken when applying it as to not use too much. It can shrink when applied too heavy. It is the best all around primer for applying over plastic body filler and for surfacing your work. If used properly it provides the proper film thickness under top coats and is the perfect substrate for bs/ss and SS.

- Easy to apply, and sand.
- Applies smooth.
- Fills well with minimum of shrinkage

- Contains Isocyanates.
- Should always use an etch primer under it.

Epoxy primer (2K)

Epoxy is a good corrosion fighter. It is has a very sticky resin and will provide good adhesion to MOST substrates. It typically has poor filling and sanding qualities (that sticky resin makes sanding difficult) . It is ideal for use as a “primer/sealer” on bare metal that requires no surfacing.
Perfect for frames and components, radiator supports, items that are sandblasted and you only need to prime and paint. You use it as a non-sanding “primer/sealer” and then paint right over it.

- Good chip resistance (it isn’t as hard as a urethane)
- Perfect for a “primer/sealer” over bare metal.
- Etch primers can skipped because of its excellent adhesion and corrosion properties. (although for maximum corrosion protection apply a wash etch under the epoxy)
- Provides good base under plastic body fillers (skip the etch if you plan on using plastic filler over epoxy)
- Epoxy has no isocyanates .

- Poor sanding qualities
- Poor filling

Polyester primer (2K)

Polyester is a very specialized primer used in very small amount in most shops across the country. But when it is needed, it does a job like no other. Polyester has a huge solids content and will fill 80 grit scratches in one coat or 36 grit in two or three! Urethane for instance provides about ½ or ¾ mils per coat while polyester can give you as much as 4 to 6! Because of it’s high solids, it shrinks very little. It is basically like spraying polyester putty. Look for a manufacture that has a recommendation to apply etch primer under it. I see NO reason to use polyester on a straight panel. It is for use only when you need some serious filling and surfacing.

- VERY high filling
- Low cost

- Very high texture
- Harder to sand than a urethane
- Possible need to purchase a large gun to shoot it.


“ISO FREE” is a urethane type primer but without the harmful isocyanates that a urethane contains.
The problem is ALL refinish products should be used with the same care and concern for your health and others. ISO FREE is like “low tar” cigarettes, don’t kid your self, it is still VARY harmful.

- Isocyanate free
- Smooth, easy sanding
- Good filling
- You need an etch over bare metal before it.

Basic tips… Etch primers can be skipped on spots of bare metal smaller than a dime or so when using all primers listed.
Most “quality” 2K primers need NO sealer before top coating with bc/cc or SS when applied properly.


All RTS or 1K sealers should be reserved to VERY low end jobs to save money. They do NOT offer the benefits of a 2k, PERIOD.

Reasons to use a sealer:

- Makes up for “some” poor preparation
- Provides a uniform color for better coverage when you apply paint.
- Helps with providing a uniform substrate for paint.
- Helps provide a better substrate when painting over a 1K primer.
- Can Help with “covering” poor prior repairs

Under collision repair conditions a shop may use sealers on every job as an “insurance” protection against problems. In a restoration environment where complete panels are primed with a 2K there really is no need to use them.
If you have chosen to use a sealer there now are a few more choices to make. First, you need to decide what kind of sealer to use. As I mentioned in the beginning, RTS or 1k could be used to save money. Why put a 1K sealer over your 2K primer (I hope you are using a 2K primer) then apply a 2K top coat? It is like the old saying, “It’s only as strong as it’s weakest link”. If you use a 1K sealer in this fashion it is like replacing a link in your tow chain with a nylon tie!

With 2K there are a few options, epoxy and urethane being the most popular. I don’t feel that there is a huge difference in the two as far as how they apply or work. Epoxy is more forgiving with sensitive substrates. It really comes down to what you feel more comfortable with. The epoxy has no isos so that would be one reason to choose it.
Now that you have decided what sealer to use you have to decide on what application.
Most sealers give you the option of a “wet on wet” (or very close to it) or a full “barrier coat” application.
The difference being with “wet on wet” the sealer is applied and then allowed a short flash time before the basecoat or SS is applied. A “barrier coat” is where the sealer is applied, then allowed to cure or at the very least to totally flash. This allows the sealer to become a barrier so the solvents from the color coat can’t penetrate it and attack the substrate.

The barrier coat procedure allows for he sealer to do MUCH more of what you choose to use a sealer for in the first place. The choice is made taking into account a few factors. How sensitive is the substrate? Or, how aggressive are the solvents in the color coat that you are applying? If it is very hot weather and you are using a slow solvent in the color coat to help it lay out, you may choose to use a sealer because you know that the substrate is sensitive and the slow solvent will attack it.

Well, that is about it for the basics, have fun!

BarryK 07-22-2004 08:59 AM

[ Epoxy has good corrosion resistance, much better than urethane primer. However, not as good as etch. So if you apply the etch before the epoxy you get the best of both.

Got this a little back wards here.
Some of the weakest epoxy;s made in the automotive market will
have at least 4 more times corrosion Resistance than the best acid etch and on the high side is 14 -16 times.

I know S&W brain washed you with this because they want to sell products (mo money!) with properly prepped metal epoxy is all you need and the strongest. Dupont and S&W did love their acid etch but its just plain outdated with todays systems.
Dupont learned the hard way because they sell so many restorers and are now taking the high road with epoxy, as they say the wave of the future! Where have they been?

Also you are right as 2K primers do poor over metal but it may interest you to know the very best of the 2ks do equal the acid etch within a few hours on the salt spray test but still bad in the 60-70 hour range at 30% before creeping.
S&W did have the best acid etch out there as far as the salt spray test as I worked hard years ago to get mine as good.
I discontinued mine about 3 years ago. Just could not sleep at night!

MARTINSR 07-22-2004 10:52 PM

Barry, I believe everything you say, but it does fly in the face of what I was taught. A couple of things don't add up.
First off, why do all the major manufactures still have etch primer as the base for their life time warranties (at least the last time I looked)? They are putting HUNDREDS of MILLIONS of dollars on those warranties. They do the same salt tests, why in the world would they do that?

I used to have OEM requirements for repairs on their vehicles to maintain the OEM warranties. I don't remember all of them, but many had a requirement of etch primers. Getting your paint system oked by the auto manufacture is no easy task as you may have found out. They REALLY test and retest these products (or do they just study the paint manufactures test results?) and getting a product on there is not easy. In fact, most products on the list were the previous generation or older!

How are these products salt spray tested? Did they have protection over them? In other words where the primers top coated with a urethane SS or something like that? Where the etches top coated with a urethane primer and then urethane SS or similar?

What about etch under the epoxy, how does that stack up?

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