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Most urethane primer surfacers are specd for use over 180 grit scratches so you will be safe at that, some users will primer over 80 grit and report no problems-I say why take that chance? My filler work gets finished to 180 at a minimum and oftentimes I'll go as fine as 220 or 320 and give it a rubdown with a red scotchbrite. It takes very little time to guidecoat and step down to a finer grit-minutes usually.
 

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Sanding with 80 Grid

I am a body man by trade. In vocational school 25 years ago, we used 80 grid dry prior to primming. Further, we sanded primmer with 80 grid dry until flat. Then, we went to progressive finner grids as needed for the top coat paint. The final prime must be done with an fine paper to prevent sand scratches on the top coat. If you epoxy prime the final sand, 360 grid dry will be fine enough for urethanes. I am taking somewhere in the 500 grid dry range or finer for urethanes without an epoxy prime. I use dry for everything prior to top coat paint.

I am still doing this today. After 25 years of experience doing this, I have not had a problem that I can attribute to using 80 grid paper the way I have explained above. I have had sand scratch issues when getting lazy on the final sands. I am very active with body work and have painted hundreds of cars over the years.

The reason you want to use the coarsest grid possible is to speed the sanding steps. I am giving away all of me secrets to getting body work done fast. It does not hurt to use finer grid. It will just take longer with no end benefit.

I hope this helps.


Tom Meyer
 

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tmeyer said:
The reason you want to use the coarsest grid possible is to speed the sanding steps. I am giving away all of me secrets to getting body work done fast. It does not hurt to use finer grid. It will just take longer with no end benefit.
I hope this helps.
Tom Meyer
I agree with this..then when sanding the "ground coat" (ground coat is last coat of primer before the base) Go to a 500/600 paper to take off any nibbins or trash..I call it check sanding to see if there is anything to fix and then shoot the base..

Sam
 

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I always used to cover 80 grit with 2k.. I had some sand scratch issue's over the years doing so until I switched to spi high build 2k.. I have covered 24 grit scratches with it, stepped the paper all the way to 400 and then topcoated.. Not an issue one.. I would not do this with a lesser primer though.. Every product is different depending on who makes it, what filler they are using to make the build etc.. There is more to it than most people think there is as I am told.. Usually cheaper 2k's will perform just fine as long as fine grits are used, but when you go with the heavier grits as I have mentioned above is when you run into problems.. Just my 2 quarts worth and not the way it is taught is schools or how most of the pro's here do it
 

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Will your primer cover 80 grit or 36 grit filler scratches without any shrinkage problems?-this is all relative to cure time, temperatures, and how the primer is applied. You can take the best urethane surfacer and rush the application with little flash time between coats, or not let it fully cure due to time constraints or temperatures and it'll definately be prone to shrinkage problems. I've done the prime over 80 grit many times over the years usually without problems but I've seen other people's work that have regular problems doing it this way. Nowadays I say why risk it when it takes such little time to step it down to 180 or finer.

Shape your filler with 80 grit then dust on some guidecoat and resand with 180 and see for yourself-it only takes a matter of minutes usually. Well worth the extra insurance that shrinkage problems won't rise up down the road. Just my opinion-do it how you like but be careful what you recomend not knowing what primer and what the capabilities are of the user.
 

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The less you ask of your primer the better off you will be. Sure it may fill 24 grit scratches but it all shrinks to some extent. Priming 80 grit scratches is asking for trouble in my opinion.Theres no reason for it.How hard is it to knock down the 80 scratches with 120 or 180? 120 i can live with, 180 is better. I will usually finish my last coat of the filler or glaze with 120 then 180 then a quick rub down with 220 and the surrounding area including feather edges with 320.
 
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I have done body work for more years than I care to remember. I know what works for me and the rpoducts that I work with. Working at home, I will regularly shoot SPI HB primer over 80 grit with no problems. I do nothing in a hurry at home, so there is plenty of flash and cure time, and I have no shrinkage problems. Before final priming/sealing, I sand with either 180 or 320, depending on what ever is handy at that time. At work, where everything is rush rush, I have to work differently. We use SW products there, so the primer is shot over 180 or 320. I always apply a minimum of 3 coats before I sent the vehicle to the paint shop. At this point, I have yet to have a problem with sand scratches showing up on my work. On the other hand, there have been many jobs of other techs that have had problems, mainly because of trying to cover 80 grit with the wrong primer, in a hurry.

If you have limited time for the primer to cure, and are short on your flash times, definately sand with the finer grits. That will reduce the need for heavier build, and reduce the chance of shrinkage showing on the final product.

Aaron
 

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I allways finish block in 180 before prime , then block primer down with 180 before final prime .Thats why its a hobby to me and not worried of the time it takes. I first sanded body work at age of 14 with brother in law . Went on and managed dealer body shops for 9 years . Still could not work it for a living would have killed the joy of hobby. Prep work for paint is the total outcome of quality . Give it the time needed and you'll be doing 10K paint work in a garage.

Trick for home paint hang some garbage bags in garage. They act like a magnet for dust particles and will keep paint work really clean of dirt.
 
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