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I recently had my car's rear end (differential and axles) sandblasted to remove 40 years of surface rust in preparation for priming and painting. Now that I've got it primed, it is very easy to see all the pitting caused by the rust.

What would be the best way to fill in all the pits before I paint it? Would a high-build sandable primer be preferable and easier to sand and finish than using glaze-type filler? I want it to look new when finished, but don't want it to be more of a chore than it has to be.
 

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Most everybody uses the fiberglass based body filler and sands it like finishing a fender. Regular body filler works, but I have heard of some who have had it come loose. (bad prep?) Heavy primer/filler tends to shrink and reveal all the dimples later.
 

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I had my '57 Ford 9" sand blasted to near white, used Dyna-Glass filler, spot putty, then painted it with a single stage enamel. I've done others the same way with sucess though would now use epoxy primer first before the filler, then 2K and paint like I would the car body.

Dave
 

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I redid a counter weight on a forklift a while ago now. We had a few redone but they used filler to smooth them out and didn't last. Chipped then rusted then lifted. I layed down about 6 coats of epoxy. Two wet coats at a time. Gave them a good couple of days to cure. Sanded down and reshot a couple of more till it was smooth. I may have done this actually four or five times. Its been a while now. I was in there last week end there are some scrapes and the paint wore off in spots but no rust and looks really good for the abuse a rental gets. I would do the same to your diff. You just need to take your time and let each coat cure before you lay on another. Don't sand it till you are ready to give it another coat.
 

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46chevyfleetline said:
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Don't sand it till you are ready to give it another coat.
What is the reason for that? I wonder because my paint guy keeps telling me to scuff my epoxy the day after spraying it [spray at night, scuff in the morning before I leave the house], even if I plan on re-coating it inside the 3 day window.
 

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BIGSKY said:
What is the reason for that? I wonder because my paint guy keeps telling me to scuff my epoxy the day after spraying it [spray at night, scuff in the morning before I leave the house], even if I plan on re-coating it inside the 3 day window.
To scuff the day after paint doesn't make sense - you are still well within the recoat/crosslink window on anyones epoxy. For SPI it's even a 7 day recoat window and it will recoat fine. My guess is that your body guy just likes to knock off any orange peel for a smoother final primed finish, but if it works for him......

Dave
 

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He didn't mention knocking down any roughness, but he did talk about "breaking the shiny surface" to let more solvents escape. I haven't been doing it because I usually don't get back to a part before my 3 day window closes on my Omni epoxy anyway, so I have to scuff regardless.
 

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BIGSKY said:
He didn't mention knocking down any roughness, but he did talk about "breaking the shiny surface" to let more solvents escape. I haven't been doing it because I usually don't get back to a part before my 3 day window closes on my Omni epoxy anyway, so I have to scuff regardless.
Suggest to him that a visit to the SPI website or, better yet, a call to owner BarryK will be in his best interest as far as using epoxy primers.
 

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If this is a custom job where you want her smooth I suggest you go over the rough casting areas with a roloc disc and stones first to remove as much casting roughness on the centersection-if it's a restoration leave as is. If there's heavy rust pitting on the tubes they can be easily filled and smoothed by shooting two coats of epoxy then allow that to cure overnight, spray 3 coats of polyester primer and guidecoat prior to sanding. Start your sanding with 100 or 120 grit then apply more guidecoat, sand again with 220-240 untill all the pits are gone, shoot two more coats of epoxy, lightly sand with 400-600 depending on color and shoot your paint. Avoid any primer buildup on casting numbers and stamped numbers by masking them off prior to the polyester if this is a resto.
 

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I am with Bob, there is no way I would be spreading polyster putty on something like a rear end when you have polyester primer! A couple of coats of polyester primer WILL fill just about anything you have.

Brian
 

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Birdguy, my method isn't the quick and easy way, but if you want it right, heres the deal.

Sandblast

sand all pock marks with 36 grit

fill the big ones with rage filler, sand

Spray the whole thing with z-chrome

sand until you can't sand anymore, then take two days off and sand some more, da a lot of it, but a lot of hand sanding

then two good coats of epoxy, I used ppg dp, but spi is less expensive, probably just as good.

Remember, if you see any spots that don't look good after the z-chrome is sanded, it will still be there after you spray the epoxy

The pictures on my web site were taken 2 1/2 years ago, and it looks exactly like that yet today

If somebody has an easier way to get around all this sanding, ask them for closeup pictures of what they did, I have tried to get close ups before of some of these frames, but nobodys really delivered.

If you don't like doing a lot of sanding, then I wouldn't go this route, I don't mind it

Anyway for what its worth

good luck

Oh, almost forgot, follow all the manufacturers specs. on temperature, air pressure etc.

Because after all that sanding you don't want to do it twice.

Rob

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