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Antique Tractor Paint and Bodywork

One more PPG Omni ?

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Dale L.

08-19-2004 06:10:06




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Does anyone know if I have to use an etching primer on bare metal first if I'm going to prime every thing with 170 epoxy primer?




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CNKS

08-19-2004 18:00:28




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 Re: One more PPG Omni ? in reply to Dale L., 08-19-2004 06:10:06  
I agree with Butch, except that the PPG Omni P- Sheet for MP 170 epoxy, that many of us use, says to use no finer than 180 grit sandpaper on bare metal for good adhesion. That simply means that you should use sandable surfacer over the epoxy to fill the sanding scratches, which you would probably do anyway.



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Rod (NH)

08-19-2004 18:50:39




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 Re: One more PPG Omni ? in reply to CNKS, 08-19-2004 18:00:28  
I have used epoxy (PPG DP) successfully over clean steel w/o a separate sanding procedure. I don't use an etch primer but I have used both DuPont's and PPG's two-step chemical etch products on panels that have been chemically stripped where I wished to avoid use of a surfacer and therefore wanted to minimize any sandscratches. Part of the process of chemically etching in this way includes scrubbing (I use a Scotch-brite hand pad) so I presume that operation also provides for a suitable "scratch" w/o a separate sanding operation. I note that the MP176 etch primer, which doesn't get a scrubbing treatment, calls for a 80-180 grit scratch prior to applying, even though it is an "etch" primer.

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Butch(OH)

08-19-2004 09:47:57




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 Re: One more PPG Omni ? in reply to Dale L., 08-19-2004 06:10:06  
Etching primers are something you will get differing opinions of but here are the basics. Each coat of paint must be bonded to the substrate be it bare metal, old paint or paint you applied earlier in the day. Chemically bonded layers are the strongest and that is why several of us here recommend staying with one manf of product and going by their data sheets for recoat times. Wait too long and you don’t get the chemical bond and you risk having peeling problems in the future. Now for the first coat of paint over any old finish or bare metal you cannot achieve a chemical bond so you have to go for second best, a mechanical bond that is accomplished by etching the surface. Etching is simply scratching the surface so the coating can "tooth in" and adhere to the surface. Any surface that has been sanded with paper 400 or coarser, sandblasted or wire wheeled is etched. Chemical etching is done with an acid, either as a separate operation or combined in a primer product. Knowing this you should realize that any old surface that has been sanded is already etched. If you have new sheet metal then you have a choice to make. My preference for etching is to sand or etch with an etching only product because I want to have an epoxy primer on the bottom of my paint jobs, not on top of a later of lesser quality etching primer. The self-etching primers were invented to save a step for body shops were time is dollars. In any case they are better than spraying over an unprepared surface. Some consider them a "can't hurt but might help" type of deal and reccomend them on everything. For reasons stated above, I use them on nothing other than I keep a few rattle cans around the shop to coat new sheet metal parts that are not going to be installed or prepped for some time just a rust protection measure.

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