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I am using Mp170 Expoxy primer on my car and I am about to paint some PPG basecoat/clearcoat over it. The 'instructions' for the primer say that if it sits for more than 3 days it needs to be scuffed and reapplied to topcoat. How important is the re-application??? I have the car sanded down with 600 grit and it seems to be decently smooth and I would reallly like to just do the top coat and not reprime it (the primer has been on for about 2 weeks or so). I am hoping that someone has painted over that primer and it has been longer than 3 days.

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I'm no body or paint guru, but the way I understand it, is it will no longer chemically bond after 3 days, so a scuffing in needed for good adhesion.
 

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it needs to be scuffed and reapplied . the topcoat needs to applied within that 3 day window . 3 days is the max, sooner if high temp. i use the epoxy as a sealer after my final sand of the 2k high build primer.i thin the epoxy. shoot epoxy let it flash off about an hour. then topcoat with my bc/cc
 

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I used SPI epoxy and it has a seven day window. But you really should take the manufactures advice.

If anything after 3 days a wash with 600 grit will remove the bug turds, and you gotta have them. I saw some just a day after spraying the high build primer. And some were pretty well bonded. Weird stuff bug urrps. Some are pin point and some are like a pen ink spots and had some height to them, JR
http://www.southernpolyurethanes.com/tech sheets new/epOXY 2006.pdf
 

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it needs to be scuffed and reapplied . the topcoat needs to applied within that 3 day window . 3 days is the max, sooner if high temp. i use the epoxy as a sealer after my final sand of the 2k high build primer.i thin the epoxy. shoot epoxy let it flash off about an hour. then topcoat with my bc/cc
i was reading in their instructions and i was thinking it went epoxy then 2k or is it the other way?
 

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Discussion Starter #6
THanks for all of the responses. The main reason I don't want to have to reapply it is that it goes down so rough and I just don't want to have to sand it down again. Do you guys think that it is 'probably OK' for a car that is garaged, never driven in the snow/rian and only driven about 500 miles per year that the top coat will stick OK? It just seems that if I re=prime it, the new primer will be over the old, so won't that have the same 'sticking' issues as the top coat? If the top coat won't stick to the cured primer, why will the new coat of primer?
 

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THanks for all of the responses. The main reason I don't want to have to reapply it is that it goes down so rough and I just don't want to have to sand it down again. Do you guys think that it is 'probably OK' for a car that is garaged, never driven in the snow/rian and only driven about 500 miles per year that the top coat will stick OK? It just seems that if I re=prime it, the new primer will be over the old, so won't that have the same 'sticking' issues as the top coat? If the top coat won't stick to the cured primer, why will the new coat of primer?
Why are you painting over epoxy????
Is the epoxy over the bare metal or are you using it as a sealer?

Products stick to themselves better then they do to other products.
Unless you are using a catalyzed basecoat, you won't get chemical adhesion anyway. You have to rely on mechanical adhesion.

With MP, if you have waited more than three days, and you just scuff it with a Scotchbrite pad, then reshoot some MP. Add 10% acetone to make it thinner and it won't go on rough.
If you have let it sit three days and don't want to recoat, then SAND it.
I would not hesitate to apply base over MP that was sanded thoroughly with 600 grit. I have topcoated PPG EPX and PPG DPLF epoxies with surfacer
numerous times after weeks of curing.
 

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so what order should these be laid high build epoxy and 2k primers?
Im no expert, technova could prolly give you better advice but here is how I did mine.

Stripped to bare metal. Metal work welding and hammer/dolly work. Cleaned with water borne wax/grease remover. Shot three coats of epoxy, 20 minutes in between coats. Plastic filler work, block sanded. One coat of epoxy to seal the plastic (not needed). Areas. Water borne wash again. Three coats of 2k high build primer. One light coat of guide coat. Block sanded till all the guide coat was gone. It did get into the epoxy a lil and even the metal in some spots. Water born wash again then three coats of epoxy to seal it all up. Prolly gonna sit like that for a year till I can get it to a pro to finish up the prep work and base coat/clear coat. JR
 

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Im no expert, technova could prolly give you better advice but here is how I did mine.

Stripped to bare metal. Metal work welding and hammer/dolly work. Cleaned with water borne wax/grease remover. Shot three coats of epoxy, 20 minutes in between coats. Plastic filler work, block sanded. One coat of epoxy to seal the plastic (not needed). Areas. Water borne wash again. Three coats of 2k high build primer. One light coat of guide coat. Block sanded till all the guide coat was gone. It did get into the epoxy a lil and even the metal in some spots. Water born wash again then three coats of epoxy to seal it all up. Prolly gonna sit like that for a year till I can get it to a pro to finish up the prep work and base coat/clear coat. JR

I prefer a solvent based cleaner such as PPG DX330 instead of the waterborne. Some techs in our area are cleaning with both solvent and water cleaners. Solvent may be restricted in some areas.

Jrouche, you are correct in your method. Sometimes you may have to use 2K primer-surfacer twice, just depends on the job.
For those painting right after sanding the 2K surfacer, there is no need to put on 3 coats of epoxy. Go right to a wet on wet sealer and paint.
I am not a big fan of epoxy as a sealer before paint. I prefer a urethane sealer but epoxy will work.
If you plan to let the car sit as JRouche is then a couple coats of epoxy after sanding the 2K will work good.
One thing to consider is that your car will be exposed to nicks, scratches, dents while you are waiting to have it painted. It will need another good block sanding to ensure that these a fixed before painting.
So, if I am going to leave a car in primer for a period of time and it will not be outside I leave it at the 2K surfacer stage unsanded. This allows me to easily block out any minor damage. If you plan to drive it or leave it outside then you have no choice than to block the 2K surfacer and protect it with some epoxy. Just remember to re-block it when you are ready for paint.
 
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