Author Topic: paint prep  (Read 10709 times)

Ricardo

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paint prep
« on: November 06, 2003, 09:22:01 »
I'm wondering if anyone can shed some light on the kind of prep work needed to prepare aluminum body panels for painting? Are there special primers that can be used for both steel and aluminum? Does the temperature of the spray booth need to be different than for steel alone, due to different expansion and contraction rates? How about heat shrinking dents etc. in aluminum, is this possible? Is it common practice to paint aluminum and steel seperately?
My car's trunk has become quite "chalky" looking, with fine crazing throughout, yet the adjacent fender paint is smooth and relatively craze free...is this due to the previous paint job not being done right? I doubt many body shops have much experience with aluminum and I've noticed several SL's with poor looking paint on the doors etc., even though they had recent paint jobs. I'd like to arm myself with some good info before I get "there".
Ricardo

Malc

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Re: paint prep
« Reply #1 on: November 06, 2003, 10:40:57 »
Hi,
Yes you need to use an "etch" primer for the aluminium panels otherwise the paint won't "stick" Etch primer can be used on steel at the same time. I painted my MGA which has aluminium doors bonnet and boot skins with the stuff.
The temperature of the spray booth really depends on the paint you are going to use, most spray booths are of the "low bake" type which is to accelerate the paint drying times.
The crazing on the boot lid could be due to a number of factors, such as age, pollution, strong sunlight etc. The fender may of been painted more recently and with a different type of paint. It sounds like the clear coat laquer may be beginning to fail, is you car a metallic colour eg silver?

The best way of prepping the panels, especially if you are unsure of the current type of paint is to strip the car back to bare metal, this had problems associated with it, mainly that of time, you don't want to strip it, leave it for weeks and then get it painted! In addition the Aluminium panels are relatively "soft" compared to steel and thus are easily damaged by power tools, grit blasting,and some chemical paint strippers although brake fluid is good! Also the aluminium oxidises quite quickly if exposed to air which must be avoided otherwise the paint will not key very well.
An alternative is to "flat" off the panels and spray it with an isolator so that you do not have a reaction between different types of paint.

Finally the paint types available vary so much I can only speak from my experience.
Cellulose-
I have mainly used Cellulose in the past because it gives a finish in character with the age of the car, it's easy to spray without special equipment and will shine up very nicely. It's not a very hard paint and thus tends to age relatively quickly. Unfortunately it is getting difficult to obtain in the UK now due to EU laws.

Enamels-
these were an alternative to the above, mainly used on Lorries (trucks) and buses, it's quite tricky to use, but like cellulose doesn't need special equipment to spray. Much harder and you can get a good shine on it with alot of elbow grease! I use it on my stage rally car as it's cheap (ish) and readily available.

Ios-Cyanates (Two Pack)
Basically like epoxy glue, mix harder and base together and spray. extremely nasty stuff and you need special breathing equipment to use it, really a job for the proffessionals, all modern cars are painted with the stuff. Extremely hard wearing and keeps it's shine with little or no attention.
My only comment on it is that it can look wierd on a classic car unless the colour is chosen with great care, to me it seems just too shiny

I am afraid at the end of the day its all down to time (equals money). To spray a car takes about 20 minutes, however all the effort is in the preperation, the better it is done the better the final job
For example the BMW 2002Tii I painted from bare metal had

2 coats of etch primer with high zinc content for rust proofing
2 coats of high build cellulose primer
flat off with 800 grade wet and dry
2 coats of high build primer (different colour)
flat off with 800 grade wet and dry
2 coats of 70% paint 30% high grade thinners final colour
flat off with 1000 grade wet and dry
2 coats of 50% paint 50% high grade thinners
flat off with 1200 grade wet and dry
2 coats of 40% paint
Left to harden for two weeks and then buffed up
Took me about 10 days in my shed!

Ops went on abit sorry!
Malc

Malc

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Re: paint prep
« Reply #2 on: November 06, 2003, 10:48:13 »
Sorry forgot to say no you cannot shrink aluminium and you must be very careful with the steel as more modern steels do not heat shrink as the composition is different after the early 70's

You can paint the two different metals in a one go with the right stuff!
Malc

ja17

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Re: paint prep
« Reply #3 on: November 06, 2003, 22:40:22 »
Hello Recardo,
Paint technology today is lightyears ahead of where it was thirty years ago. At the Blacklick Tech. Session, Bob Gecco reffered us to "Automotive Paint Handbook" by John Pfanstiehl. This is an extremely good resource on the subject. I have been doing my own paintwork for the last thirty years and still found this book to be very informative. Thanks Bob Gecco.



Joe Alexander
Blacklick, Ohio
Joe Alexander
Blacklick, Ohio
1969 Dark Olive 280SL
2002 ML55 AMG (tow vehicle)
2002 SLK32 AMG (350 hp)
1982 300TD Wagon turbo 4spd.
1963 404 Mercedes Unimog (Swedish Army)
1989 flu419 Mercedes Unimog (US Army)
1998 E430
1974 450SLC Rally
1965 220SE Finback

Ricardo

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Re: paint prep
« Reply #4 on: November 08, 2003, 18:27:12 »
Thanks Joe and Malc. I will look for this book Joe, sounds like a keeper, hopefully it will be available through Amazon.
Malc...the original paint was # 834 Moss Green Metallic, but the car was resprayed after extensive body work about 10-12 years ago with # 501 (Winerot I believe), which is a burgandy non metallic paint.
Is the etching primer you referred to made especially for aluminum?
By "flating off" do you mean  surface sanding but not down to bare metal and then a sealer coat before any additional filler or primer work?
Ricardo

n/a

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Re: paint prep
« Reply #5 on: November 08, 2003, 20:20:13 »
If you want to learn about painting on aluminum, US Paint makes a paint called Awlgrip or Alumigrip which is designed for use on boats and airplanes. There are a unique series of primers and adhesion promoters that are used to help the paint stay on aluminum and also move with it as aluminum moves around more than steel does. Awlgrip is sold through marine suppliers; if you go to a BoatUS or West Marine store they can get Awlgrip literature for you. Note that you don't need to paint your SL with Awlgrip, although it is great stuff, but the information on how to paint on aluminum may be handy.

ja17

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Re: paint prep
« Reply #6 on: November 09, 2003, 09:16:34 »
Hello,
Most major paint sytems  have these "self etching" primers. They work extremely well on aluminum. They are sometimes called "wash primers" and contain an acid which actually etch or eat into the surface slightly in order to form a tough bond. They also contain a lot of unhealthy to breath chemicals like zinc chromates etc. (be sure to use a good resperator in a well ventilated area). Usually  a very thin transparent coat is applied. Standard primer surfacers can be used on top of the this primer before paint. Many modern two stage primer/surfacers work fine on all materials by themselves and do not require these "Self etching" primers to work.
My advice is when you figure out what paint system you are using follow the manufacturer's instructions for preparation and primers on different materials, You are more likely to have a bad experience mixing incompatable paint systems than having adhesion problems with todays paint coatings.


Joe Alexander
Blacklick, Ohio
Joe Alexander
Blacklick, Ohio
1969 Dark Olive 280SL
2002 ML55 AMG (tow vehicle)
2002 SLK32 AMG (350 hp)
1982 300TD Wagon turbo 4spd.
1963 404 Mercedes Unimog (Swedish Army)
1989 flu419 Mercedes Unimog (US Army)
1998 E430
1974 450SLC Rally
1965 220SE Finback

erickmarciano

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Re: paint prep
« Reply #7 on: November 09, 2003, 20:20:12 »
1- bare metal
2- etch primer < use the 2 best products on the market   Picklex-20 or Zero Rust >
3- epoxy primer
4-2k hight build primer>
5- base coat
6-clear coat

this way the car will NEVER RUST

1971 280sl
1962 VW
1954 Allstate vespa
1958 gs150 vespa
1962 gs160 vespa
1994 E500 W124
1971 280sl
1989 Porsche 930 coupe
1988 e30 m3
2001 ducati mh900
2006 ps1000
1962 Vespa GS160

Malc

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Re: paint prep
« Reply #8 on: November 13, 2003, 10:23:09 »
Ricardo
Flating off - yes this is sanding between coats to make it as smooth as possible, but provide a "key" for the next layer of paint. Use plenty of water with a dash of soap, don't use washing up liquid or detergent, try and get some natural soap as the other stuff has lots of salt and other chemicals in it. Use a rubber block when you can while sanding too. Yes try and avoid going all the way through back to the metal!
I use two different coloured primers so that I could flat it off and any unseen imperfections become apparent, which you can then deal with using cellulose putty, filler etc as the mat finish of the primers makes it difficult to see them, but a gloss coat will show them.

Primers are available for ally panels, see other posts

Each coat acts as a sealer for the one below however make sure you stay with one particular paint system from start to finish otherwise you may get a reaction between paint types

Fianlly sounds like the repaint of your car some time ago might not of been that great hence the failure of it, best to strip it all off rather than risking going over it.

Hope this helps
Malc


Ricardo

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Re: paint prep
« Reply #9 on: November 15, 2003, 08:22:47 »
Thanks again Malc!
Perhaps you or others can recommend what kind of paint remover one would use for automotive paints? I have stripped lots of furniture with regular paint removers (mostly methyl chloride),but is this the type to use on enamels or laquers? People don't really use brake fluid do they? Hate to think of using regular strippers indoors with the noxious (toxic)fumes they produce, I know ...respirators and exhaust fans...thought there might be something like brake fluid that would be a little friendlier...dreaming I suppose.
Richard

n/a

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Re: paint prep
« Reply #10 on: November 16, 2003, 11:18:53 »
Ricardo,
I used an airplane/automotive paint stripper from my local Sherwin Williams Automotive paint supplier (Look in yellow pages under automotive paint distributors) It worked very well, although it did smell. I also used the identical product purchased through Eastwoods, and they now have a non-toxic version, although I have not tried it.

Malc

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Re: paint prep
« Reply #11 on: November 16, 2003, 17:03:53 »
Richardo,
I actually avoided using chemical strippers as they tend to do strange things to seam sealer etc, amd I didn't want anything effecting the paint. In the end I used a combination of sanding disks on an angle grinder, hand rubbing and grit blasting, I built a "tent" in my garage and blasted the car, the good thing is it's "dry" so little chance of water making more rust! Blasting advantages are you can get in all the corners etc, however you have to be really careful as it can distort ally panels.
Slow, messy and you must wear a respirator or very good dust mask, however you get back to bare metal, blow away the rust, and have hours of fun welding! and provide a great surface for the paint.
Finally I actually built a "barbeque" spit out of 50 quids worth of steel so I could spin cars through 360 degrees, makes working on the bottom so much easier, that horrible old underseal, use a blunt needle gun to chip it all off.
Ain't car restoration fun!
Malc

tuultyme

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Re: paint prep
« Reply #12 on: January 20, 2004, 21:53:17 »
Where is there a source of Eastwoods products here in the states?

Bruce; Blaugrun(green) 1970 280SL; IL

TA250SL

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Re: paint prep
« Reply #13 on: January 20, 2004, 23:12:22 »
You can purchase directly from their web site at www.eastwood.com.

Tom

70chevelle

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Re: paint prep
« Reply #14 on: January 21, 2004, 15:42:14 »
Whatever you do, use the proper protection.  Some of these products don't kill you immediately, they can fester in your system for years until they effect you.  If you can afford this type of car and the associated costs of parts etc, spend the $300-$400 for a fresh air system.  This is not to imply that  laquers (cellulose) and catalyzed enamels are not harmful because they are.  Respirators will filter the toxins for all three, the problem with iso's are that they have no smell, so if your cartridge is bad, you won't know unlike the laquers and enamels.  If you choose to use a respirator use fresh cartridges every time you spray.  Once they are exposed to the air, they have a life of approximately 8 hours. Hobbyaire and Neoterik make some very nice and affordable fresh air systems.  Just an FYI.

70 Chevelle 13.7 @ 99.7
70 280 SL Silver/Black top
99 C230 Kompressor
03 Dodge Durango SLT