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Old 04-09-2012, 08:35 AM
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MARTINSR MARTINSR is offline
Brian Martin,Freelance adviser
 
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Quote:
Originally Posted by Lizer
Brian, I've read those articles before...I'm not using much down pressure so as to push down on the panel. These are narrow low spots anyways, ones that couldn't be created from pushing down on the panel.
ANY pressure can do this. I am talking it can be so touchy that you literally have to hold the sander so even it's own weight isn't on the hood as you run the sander over it. This sounds goofy but it's true, you literally need to "fly" the sander over the roof tops without landing. This is where NEW, QUALITY, SHARP, COARSE paper is so important.



Quote:
Originally Posted by Lizer
I'm using new sandpaper, but of course as you can see it's not a very course grit for blocking. What grit are you using to cut it down?
I am a big believer in CUTTING the filler flat. For this you need the coarsest paper possible for each step. Cutting that filler FLAT on a flexing hood, 40 grit if you must, heck 36 if you must. Now I don't use anything coarser than 80 anymore after some study of the subject back a while ago involving chatting with the Evercoat Tech line. But on a flexing hood, darn yes 40 is going to be used.

The polyester primer will fill a LOT and possibly all you need, it's hard to say being we can feel your hood. But realistically you are going to likely need some "bondo" first. This is where that 40 grit NEW, SHARP, QUALITY paper comes in, and CHANGED OFTEN. We are talking changing so often that you put the "used" paper back in the box for the next job where the panel isn't flexing. You hardly use it at all, and change it, why not? The SHARPER, NEWER, QUALITY paper you use the better, the more it will CUT without flexing the panel.

Personally I treat the polyester primer the same as polyester putty skim coat. I ONLY use it when I KNOW, I KNOW that it will be the last application.

I use it once I KNOW everything is filled and it's FLAT, I use it as a skim coat over the entire area to fill tiny imperfections and pin holes and sand scratches. At that point I am not trying to cut it FLAT, that has already been done with the COARSE, SHARP, NEW, QUALITY paper, it is FLAT but full of scratches too coarse for primer, like those 40 grit scratches. So polyester primer is applied over that to fill them.When sanding the polyester primer I go to 120 and even 180 but 120 scratches are pushing it, but quality urethane primer will handle them if you don't bomb the primer on and let it flash well between coats.

I can't emphasize enough that when using filler on the rest of the car where the panels aren't flexing you use finer paper and never ask the primer to fill so much, you don't need these coarse papers so much at all. But on that flexing hood, you need to go way overboard with paper, and lack of pressure so the paper does the cutting, and for this you need a coarser grit.

Brian
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