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Old 07-25-2005, 06:05 PM
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Epoxy vs Urethane Primer

I've just discovered this bulletin board, and although I am not a hot-rodder, I hope you won?t mind my asking some questions.

I am restoring a seriously rust-damaged MG Midget. The running gear and all components have been removed from the chassis. I've replaced all of the damaged metal with new steel and I am getting ready to apply primer. New metal is either galvannealed steel, plain steel sheet or black primed replacement panels. Remaining original metal has been chemically stripped to bare. Everything has been sandblasted (including the replacement panels) and then chemically etched with Eastwood's Oxysolve which leaves a dull-gray zinc phosphate coating on the steel. (This description applies to all interior and underside surfaces. Exterior surfaces have not as yet been touched and are either original lacquer or black primer, which will be removed prior to refinishing.)

I have nearly zero automotive refinishing experience so I asked my paint supplier for advice. I have read in several places that epoxy primer should be applied over bare steel. The paint supplier said that is no longer true and that I would be better off using a urethane etching primer on bare metal, followed by a 2K high build primer. He said that the new urethanes are better than the epoxy primers. I ended up leaving with Western Automotive Refinisher's Select RS1000 Etching Filler and RS3702 2K HS Primer.

I have the engine compartment primed with the RS1000. It applied easily and sands very nicely. I am moving on to the interior of the tub and underside of the chassis. Checking this bulletin board I see that it has been recommended in several places to start with epoxy and then 2K urethane primer.

Any comment on starting with a metal etching urethane primer such as the RS1000? This has been way too much work to mess up the paint at this stage of the game.

Thanks!!
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Old 07-25-2005, 06:39 PM
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talk to barry k in this forum hes got an spi epoxy prime that is excellent,especially against corrosion, do a search spi epoxy prime nothing but good things about this product,mike
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Old 07-25-2005, 07:24 PM
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I second the SPI epoxy, great stuff.

Vince
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Old 07-25-2005, 08:50 PM
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Never trust the guys behind the paint counters ... only rarely will you find one that is competent in this field and works in it as well.. I think you should go with the epoxy primer.. Etching primer is yesterdays technology and the epoxy is superior... If you are looking for quality products,, and I think you are, you can not beat that of www.southernpolyurethanes.com
Go there and check out the products as well as prices... If you want to purchase something then you will need to go here and get the 800#
www.bakerpbe.com .... I make mention that they also carry HOK and just about anything you need for your automotive needs.. tape, paper, paints etc... and their shipping is first rate and cheap... Hopefully Barry will chime in on the primers for you
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Old 07-26-2005, 07:04 AM
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An etching urethane primer? Never heard of such a think? If you've got all of this metal sandblasted this would be the ideal texture for epoxy primer, some self etch primers are not recomended for use over blasted metal- trapped acid is a possibility.

I'd use two good coats of epoxy then either a polyester or urethane primer surfacer. Polyester primer works great in pitted areas or anywhere there is a good amount of fill needed. Hopefully Barry will jump in and identify the
urethane etch primer you've described.
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Old 07-26-2005, 10:50 AM
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I too have never heard of such a thing as a urethane etch primer. And fill. Not any etch I've seen. All I've used are very thin. I am guessing some information got mixed up. Etch primers or wash primer that I've used has been two parts and is an acid. It basically is the same thing is metal prep except sprayed on instead of wiped on like the metal prep is. I would go with the epoxy also. The recommended procedure by icar when I started was to metal condition or apply an etch primer to all bare steel followed by an epoxy primer then a filler primer if needed, but I always skipped the etch. The only time where I've seen etch primer used alot in bodyshops or places I've worked that worked with metal parts is the often used etch primer on aluminum parts. If I remember correctly, you are not suppose to apply filler over etch primer. It would need to be epoxy primed first or any filler work done before the etch is applyed.
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Old 07-26-2005, 11:01 AM
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I don't see where anyone said etching urethane primer?
Said etching primer such as acid etch primer. (yesterdays tech)
Big difference.
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Old 07-26-2005, 11:11 AM
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First post Barry
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Old 07-26-2005, 11:32 AM
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Just not enough coffee today, we will correct that right now!
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Old 07-26-2005, 10:26 PM
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is it possible he's talking about some of the newer dtm urethane primers like barry's turbo primer or rm power fill, etc??? i think just about all the paint companies have atleast one by now.
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Old 07-27-2005, 08:58 AM
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I was under the impression that this is urethane but the Tech sheet I have does not say that specifically. The description reads: " RS1000 Etching Filler is ideal for spot/panel repairs where bare metal is present. It increases both adhesion and corrosion protection on bare substrates. It fills minor surface imperfections and may be topcoated directly for quick easy repairs." It goes on with other data, but it does not give any chemical composition. It is a two-component paint.

This is suppose to be a newer product technology that is better than epoxy, but that is according to the paint dealer.

<<<Never trust the guys behind the paint counters ... >>>

That is why I am asking. This stuff goes on great and it sands very nicely, no loading of the paper at all. It was recommended that I use the RS1000 for the base and R3702, which is a high build primer surfacer, over that.

I've put in hundreds of hours rebuilding the monocoque chassis and I don't want to dork it up now. I'll shoot myself if the paint starts falling off in a year. My main concern is corrosion. Since I have new metal/clean metal I want to apply the best protective coating I can.

Maybe epoxy is a better way to go. Thanks for your insights.
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Old 07-27-2005, 11:36 AM
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Well I have had great results with epoxy over bare metal and then all the body and fill work on top of that. I use PPG DP series epoxy and the results have been stellar.

But, it's designed to be applied over certain finishes including bare metal. I dont know anything about Eastwood's Oxysolve. So I think you are going to have to get a hold of a knowledgable rep and talk about it. Forget the jobbers.

It has been said over and over on the forums and I have really come to believe it.

Step 1. Pick your supplier and your final paint choice first.

Work backwards from paint to sealer to fill primer, etc until you get to the material you spray on bare metal. Stay with the same manufacturer.

Wish I could be more help but its hard to talk compatibily when you are using different manufacturers.

Rich
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Old 07-27-2005, 11:07 PM
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If you have already converted (phosphated) the bare metal then an epoxy primer would be a good choice. Most epoxy tech sheets show this as a preliminary step to applying the epoxy primer.
Epoxy primers come in different varieties and today's formulas are usually lead-free (LF) or chromate free. The absence of lead and zinc chromate in the filler materials used in epoxy and other primers is safer but lessens the effectiveness of the sealing capability and moisture/rust inhibiting quality of the product. There are still some good protective epoxies with good solids content available-but most of these are found in industrial or aircraft finishing products.
Rust/corrosion inhibitting does NOT mean rust nuetralizing or proofing. It means that it inhibits (slows down) the process of corrosion.
Lead and zinc chromate are added to primers and paints because of their dense and large molecular structure which helps make a barrier against moisture and UV thereby helping slow down rust and corrosion. Because of environmental and health concerns, fewer primers and paints contain these products.

If anyone has some good high solids epoxy primer information that they have good experience with, please add the name and company to this post. I have looked at tech sheets for some Western Automotive/Sherwin Williams finishes and Dupont epoxies in the medium build range. Phillips Industrial (PPG I think) glass filled epoxy is another interesting product I'd like to try.

Most of todays DP and other epoxy primers/sealers are lead free and sometimes chromate free and low in solids. They don't have the sealing and rust inhibiting qualities that were available in the original formulas of 20 to 30 years ago. 20 years ago PPG had better high solids epoxies in their aircraft line-DPU35 for one, and another I can't remember the part number for. These time frames are when putting body filler over epoxy primer started to evolve as a technique that was still not thouroughly proven as a surefire effective method of corrosion control for bare metal under filler. Also it was reccommended in the tech sheets for most epoxy primers, that bare metal be prepped with metal conditioner/treatment and cleaned prior to the epoxy being applied.

Epoxy does NOT nuetralize rust.

It will work to put body filler over epoxy primer and also most 2 part high solids catalyzed primers as long as the primer film has cured and released the solvents/thinners in it and is not affected by the styrene in the filler and the primer is scuffed/sanded for mechanical adhesion of the filler. Most of todays fillers/bondo have new resin technology that gives the adhesion a level comparable to epoxy glue. All body fillers and primers "stick" by mechanical adhesion. Self etching primers "stick" by mechanical and chemical adhesion. The phosphoric acid in etching primers can drastically slow the cure of body filler/polyester resin so this is not reccommended for use under filler.

Phosphoric acid will also affect the activator/hardner of some epoxies.

Etch primers are very good for what they were designed for and come in different application varieties.

1: Mild self etching primers with some solids and filling/sanding capability which can be used OVER body filler.
2: Vinyl wash primers with a more aggressive etch/acid content ratio and a VERY thin film build. These vinyl wash primers need to be primed and topcoated over and are mainly used instead of using a metal prep/phosphate coat and should NOT be used over body filler.

Always consult the manufacturers tech sheets. When using metal etch products (acid based) always use caution concerning the possibility of acid residues left in the film before applying something on top that could be affected by the acids such as:epoxy activator/hardner or polyester/fiberglass resins.

Sealers for application between the undercoats/primer and the topcoats/paint are usually limited in how effective they are at stopping solvent penetration, especially when the paint is applied wet on wet to the sealer. If you can take a rag with solvent from the paint and wipe off the fresh sealer coat even after 30 to 60 minutes, how effective are they at sealing. Most of these are really "tie coats" that help bond the 2 layers together. Most basecoat paints are actually better "sealers" when it comes to solvent penetration due to the parafin used to orient the metallics and pigments, but these will still let solvents thru. The best "sealing" is accomplished by good quality 2 part high solids primers or fully cured 2 part paint finishes. Tie coat sealers or epoxy sealers/primers are still a good part of a complete refinish system.

overspray
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Old 07-28-2005, 09:16 AM
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Overspray if you want the name of top notch company sure... SOUTHERN POLY URETHANES. As far as your post, I think it will and can be very confusing for any newbie or anyone for that matter.

As far as prep for above companys epoxy primer>>>>>
"Metal or Aluminum must be clean of all rust and oils and any films. Never clean metal with lacquer thinner or reducers of any kind. Raw metal of any type is always cleaned best with a waterborne wax and grease remover such as SPIís part # 700-1. This is most important for prepping aluminum. Sand with 80 da paper or coarser ONLY."

I as well as countless others on this boards and others I frequent use these products and never have problems...

Matthew
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Old 07-28-2005, 02:52 PM
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Matthew, I like what I see at the SPU website. Their tech sheet says to put the body filler on the wet epoxy primer. I used to be a tech rep for a filler company and the chemists said that was a no-no. Solvents in the primer could cause problems or loss of adhesion between the filler and primer. On Dupont's website the epoxys they recommend under filler specifically say to dry overnite (16hrs) or force dry/bake at 140F for 30 minutes.

Yeah, I can get pretty technical, but i can still speak hotrod. Now days I mostly deal with rusty old bodies and I prefer the etching primers on a lot of that type of work.

overspray
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