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Join Date: Mar 2009
Location: Newtown, CT
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Repair rust bubbles on DD

Getting ready to start my first small paint repair, had a couple questions.

Repairing a rust bubble around the rear fender, probably 1"x2". I've done some research online, and talked with the guy at my local Sherwin Williams shop. Going for a decent looking repair, but keep in mind the car ('04 A4) has 160k miles on it and worth at best $4-5k. I don't have a large air compressor, so will use a preval sprayer.

Plan is to:
Remove rust using grinder with wire wheel.
Fill any pitted areas with bondo.
Clean with wax and grease remover, then tack cloth.
SW self etching primer.
Ultra 7000 base coat & clear. Tack cloth in between coats.
Wet sand and compound.


A couple questions:
-SW said a rust converter or Ospho type product was not necessary, but reading online I think maybe its a good extra step.....agree?
-Should something more agressive than wire wheel be used? I'm not totally clear how much material I should be attempting to remove.
-I've read online to use epoxy primer over etching primer, but directions on the stuff I bought say it is OK to put base directly to it so I will do that.
-I'll tape off the area I'm going to paint....including some area (2-3") beyond the rust that I'll just scuff not grind in order to make a reasonably good transition. Any further blending effort I think is beyond my skill level.
-No sanding after primer or base coat, correct?


I'm looking forward to giving this a shot. If it goes well, I'd like to get to the point to do the bumper and decklid on the 911. If it goes poorly, I'll send it to the shop and not much was lost!

Thanks in advance.
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Old 04-14-2014, 10:01 AM
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While still not like cutting out the underlying damage, at least it should slow down. I would also do a rust treatment before the primer. You should use a 400 wet between the different coatings to give the paint some bite.

Also, be aware that Preval sprayers have a very narrow spray pattern, so go really slow and light when you apply. Do many thin coats with dry time between to avoid runs.
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Old 04-14-2014, 12:04 PM
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FWIW,

When I did this type of repair on my Bus, it was wire wheel to remove chunks, rust neutralizing spray to slow the rust, high build primer, 400 wet, high build primer, 400 wet, several coats of single stage top coat (PPG Omni MTK with Hardener), compound, wax.
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Old 04-14-2014, 12:11 PM
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Thanks Harry, just what I was looking for.

I'll plan to add the Ospho prior to the primer.
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Old 04-14-2014, 05:10 PM
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I would check to see if you can use ospho under your epoxy primer .
most epoxy primers will not stick to ospho type products .

a wire wheel will not remove the rust for the most part it will only take off any heavy rust flakes .

in my shops we would hardly never use a rust product like that unless it was down the insides of frame rails or other places that you can not get to .

you will also want to use a filler type primer over the top of your E-primer you pick .
no matter how well you feather edge out the top coats and undercoats to the metal substrate the mill build of e-primers will not hide the feather edges . a month after you are finished you will see your work ringing out .

as for adding a product to another product like what your saying . that is a big NO !
most ospho products are water based and most primers are solvent based , not a good match !

the best way to do the repair would be to spot blast the rust with one of the cheap 30 dollar spot blasters . this way you will remove the rust once and for all and not try to cover it .

then spray one coat of e-primer over the bare metal . then do your plastic work .
I would not pick bondo as the type of plastic filler . there are many better brands other then bondo brand plastic body filler out there .
bondo is a maker of body fillers not the name of the product .

once your finished with you plastic body filler work spray 2 to 3 coats of a filler primer over the body work .
once that is dry sand with 400 or 500 grit paper .
you can then go right to a top coat ( base / clear )
if you find you get a little dust in your base color you can let if flash and sand it lightly with a ultra fine paper like 1000 or 1200 to remove them .
if you go with a courser grit like 400 or 500 you may see sand scratches later on .
then go spray your clear once your happy with the way the base color looks and is blended off .
if your spoting in the clear and not painting it right to a edge of the panel you will want to melt in the edges of the clear to the old finish by using a very over reduced clear on or around the edges of the new clear .
you could also just use a blending reducer for that .

once it dry you can then buff the area you repaired to bring a good shine back to the edges of your blend area .
Old 04-15-2014, 07:16 AM
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JEM JEM is offline
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sherwin williams paint is jjunk i would not put it on my mail box

every shop in this area that used it has stoped using it too many come backs paint peeling off


jery
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Old 04-16-2014, 12:42 AM
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the sad fact is all manufacturer's paint systems will work if you follow the TDS / TRM (technical data sheets) / ( technical reference manual ) .
the solvent based ultra 7000 line has been out for years and years .
on the other hand it would not be my 1st pick to paint with !
all top auto makers use BASF for there coatings for good reasons .
what it comes down to is cost and convenience to the end user .

i'm sure you picked S&W because they have a store right in Danbury and that's fine .
Old 04-16-2014, 03:46 AM
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962 -- thank you for the detailed help!

Never considered a spot blaster....didn't realize such a thing was an option.


What type/grit media should I use?



Looks like I'm heading back to the shop (yes, SW in Danbury) to get some filler primer.


Will post an update when I get this finished.
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Old 04-17-2014, 11:58 AM
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just about any light blasting sand will do for the hand held spot blasters .
if your just doing a small little spot or two of rust a little compressor like for a nail gun will work .
the S&W guys are ok I know some of them and they are very helpful so your more then good using them .
Old 04-17-2014, 05:33 PM
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