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Sun Dec 10, 2006 6:26 pm

I have a question, I have just spent the weekend sanding off the spray can primer with 40 grit sandpaper, and have gotten it all the way down. I know that it too rough to use, but I have read from from a few websites that it is the best way to go for adhesion purposes. I primered it with MP series Omni Line MP170 PPG Primer. It's there now, and I only see a few grooves on one area. I tried to keep the paper from any rounded corners. Now will any of the groove lines or scratches left from the rough grit show up later in the paint? What can I do next? Could I have screwed it up by using the 40 grit over the whole bus? If so what can I do to really straighten it out. I really need a serious layout on how to do this restoration. Jason

Sun Dec 10, 2006 8:58 pm

40 grit is a little deep to be priming over but with a catalyzed primer you will probably ok if you can block them out. With catalyzed primer shrinkage is minimal so if you get it straight while prepping it will usually be ok. Spray a guide coat and start blocking out the 40 grit scratches. I would start with 180 Then I wet sand with 320 then to 400 and maybe 600 depending on the color you are going. You may need to repeat the process of priming, guide coat and blocking several times to get it straight and get all the sand scratches out. You'll know when they are gone.

I usually use PPG NCP 270 for priming and have had good luck with it.

Sun Dec 10, 2006 11:48 pm

Forty grit is overkill. You don't need such large lines in paint or bare metal to get the primer paint to stick. Metal etching primer would stick well to bare metal without severe scuffing. Using 40 grit sandpaper just makes for a lot of extra work. If metal is rusty, then you can knock it off with 40 grit.
I used 80 grit on my random orbital sander to start removing the old lacquer on one of my buses (it saves time), then used several higher grit sanding discs to knock the last bit off to get down to the original paint without messing it up.
Did you take it down to bare metal or down to the paint that was beneath the primer? If it is paint, then follow Durf's advice.
For scuffing up a topcoat before you paint over it to get the new paint to stick well, you want to use anywhere from 500 grit to 700 grit wet sandpaper, or lower if primer filler is going to be sprayed on it.

Mon Dec 11, 2006 8:06 am

40 Grit is harsh, it is great for bare metal when putting filler on to give the filler a good bite and removing previous filler and paint but 80-150 grit works great for primer. Now if you have filled a small ding on a good painted surface and are repainting that area then 220-320 works great for areas like that.

Mon Dec 11, 2006 3:35 pm

if you took it down to metal with 40 grit, then epoxy primed it, and now plan on bodyworking it, then highbuild primer, coarse block the vehicle (180 or 220), work out any last minute imperfections, prime, final block, seal, paint, you will have no problems.

if i'm doing a vehicle that is pretty banged up and requires lots of bodywork i take them down with 36 or 40 grit on a da and an 8" orbital (mudhog). you can't file, filler, weld, stud weld, etc. over paint, so this is my reasoning.

Thu Dec 14, 2006 7:03 am

I'm no expert but you're more experienced than I and since I'm going to re-paint my car this winter I need as much information as possible so. .. Why do you shoot that much primer? Isn't 5 coats too much?

Fri Dec 15, 2006 4:15 pm

I strip with 80 grit. Then I went to 180 and primed with a high build solids filler primer. That was followed by a guide coat and a sanding with 320 and then 600. Then it was a misting of DPLF and the basecoat, pearl layer, and then clear.

See it here: http://www.german-pride.com/Joomla/index.php?option=com_gallery2&Itemid=43&g2_itemId=568

Scott Faivre