4drgl

I am repainting my OB 928...

I am trying to confirm a current working process of painting the aluminum panels. I have gotten several different suggestions/styles of prepping/painting aluminum. I have also done my research and found it seems to vary widely how aluminum is prepped and painted.

The car is completely bare, it required a complete strip due to paint crazing/rust forming underneath cracked paint...

Strip old Paint
Repair Dents/Bondo here??? or after epoxy
Alumiprep (phosphate acid etch, surface prep)
Alodine (surface adhesion promotor)
Zinc Chromate or Epoxy Primer?

I am going to use PPG materials. So if anyone knows ppg product #'s or has information pertaining to this it would be greatly appreciated!

I can only afford to paint the car once and properly.

Thankyou, MrMerlin/928tt for suggestions and help. You have helped greatly, I am just trying to confirm the process.





Pics for reference... The last pic, the driver fender had been damaged pretty good, and previously repaired with lots of bondo... I have sourced a replacement... However the repair order or chemicals as best as I could dissect was this (reason for listing is because the paint was almost impermiable took about 4 coats of aircraft stripper to get the surface soft enough to scrape down...):

The fender was stripped
scuffed up/cleaned then bondo applied
then the zinc was on top of the bondo to entire fender
primer
another layer of bondo
primer
paint

Any other suggestions??

Fabio421

928tt does this for a living as well as being a sales rep for the products. He knows what he's talking about.

Giovanni

SPI www.southernpolyurethanes.com has one of the best epoxy sealers in the automotive industry and its $70.23 for a gallon of epoxy sealer and $70.23 for a gallon of epoxy activator. It gives you 2 sprayable gallons. This epoxy eliminates the need of acid-etch primer and is safe for aluminum. You can NEVER use there epoxy on any treated metal (acid etch primer, rust converters, soda blasted cars etc). Follow it with their high build primer. Then paint of your choice and finish up with their universal clear. Again, their product is top notch and its probably better than PPG.

If you want to stay with PPG check out their K36 epoxy.

4drgl

I'm not discounting what he told me. I've researched the products that he/I discussed. I also discussed with several other people and have gotten an array of different options and wanted to know how anyone else does it/ processes they've used.

John V

painting aluminum "correctly" can be difficult especially in difficult environments (aircraft and marine especially). Automotive is not consiered to be as punishing and therefore you can get away with things that wouldn't fly in other arena's.

For years the prefered method for coating aluminum in aviation and military applications was the alodyne process. There are many "equiveleants" but it involves a chemical conversion coating and nuetralizer apllied to pristine- clean aluminum. Even more so than steel, aluminum must be prepped properly and coated or converted promptly or topcoats can fail. When I say promptly- I mean within minutes of prep.

There have been some studies by Boeing and the military which compare the adhesion and salt spray protection of epoxies in place of a conversio coating that have shown very pomising results.

I'll tell you that all I use now is the SPI epoxy primer over perfectly prepped aluminum. It's an excellent product and I have no affliiation wiht them. There are some limitations which i don't have time to get into now but it's worth a look.

James Bailey

The list of those on this forum who have actually painted a car is a very short one......many have paid to get them painted however.

JHowell37

Any specific reason you're using PPG? I'm not trying to get you to second guess yourself, I'm just curious why you'd choose them over a different brand if you're unfamiliar with product numbers.

The other reason I'm asking is because PPG has a **** website that's difficult to navigate. A couple of the other companies have websites that are a bit easier, and in their technical manuals, you can find the info you need.

You haven't even specified which line of PPG products you want to use. They have several (which is another reason they annoy me (too many product lines is confusing.)


Let's say you're planning to use their "Deltron" line.

You can skip all of the acid etch, acid wash, etc. Go with their DP40LF epoxy. It is a DTM product. This means that it can be applied directly to bare metal substrates. It can be applied over or under body filler. It can be used as a surfacer, but let me tell you this. Epoxy primers used as surfacer (for blocking) aren't very good. Blocking the stuff is a bitch.

1. Finish stripping the old paint and get rid of any surface rust.

2. Apply body filler.

3. Spray the car with the DPLF.

You'll want to buy a dedicated primer-surfacer. Someone mentioned a product called K36, and said it was an epoxy. This is incorrect. K36 is an acrylic urethane, not an epoxy. It can be used on "properly treated"(i.e. after etching, acid wash, etc) bare metal. Or it can be used over DPLF epoxy. So you do want to use this after you apply the DPLF.

4. Apply K36. Follow instructions on technical sheet.

5. After you block, you may apply another coat of DPLF over the K36, mixed as a sealer.

6. paint.

https://buyat.ppg.com/refinishProduc...c-04551702a5df

For the link above, click on view product listing. From there, you click on various products, and each one has a link under the column "TDS." This is the technical info. The technical info is the specific instructions for using the product. If you adhere to the info in these technical manuals, you may do a bit more work, and spend a bit more money then a professional. One thing that won't happen, is you won't be here asking how to correct a problem caused by cutting corners.

JHowell37

I would be one of the few who is on your short list.

M. Requin

Me also (painted cars), although not my 928 (yet), but from reading posts for the last few years, I would defer to JHowell37 on questions of application. That said, I think PPG is fine (although confusing as he noted), I would just recommend as JH did sticking with one manufacturer's product line from prep to clear- if you follow their tech recommendations TO THE LETTER you will be in pretty good shape no matter whose product you use. Pro's get away with mixing lines (esp clearcoat) but you really have to know what you're working with . And there are some good forums to help out (google autobody paint)- so have at it and good luck. Your prep so far shows you're pretty serious about the project. And pics not only of the finished job but in progress - I learn a lot from that!

M. Requin

and BTW I know absolutely less than nothing about aviation coatings and alodyne etc- let John V speak on that one!

FBIII

Fenders are pretty plentiful for OB's. It might save you a lot of time posting here for a reasonably priced fender.

John V

Ok I'm back - sorry for all the typos in the previous post but I was on my way out. I want touch on this because it is controversial and you get all kinds of opinions. I've done a bit of research, and will share what I've learned.

It's easy to coat aluminum but it's a real challenge to get coatings to stick well long term where thermal and environmental conditions stress ahdesion. Aluminum oxidizes extremely quickly- literally within minutes of cleaning. Unlike steel where even minimal rust is visible, aluminum will "look" clean when in fact there is an oxide layer forming which will impede the proper adhesion of coatings. The alodyne types of processes (and each paint manufacturer has a compatible "conversion" process) incorporates a special etchant to condition the aluminum and chemically converts the outer layer to a surface that resists oxidation longer and readily accepts other coatings with a tenacious bite. Basically- the alodyne bonds real well to the aluminum and then primers can bond real well to the alodyne. That has been and continues to be the gold standard for reliably painting aluminum in stringent applications.

Enter epoxy. Epoxy has incredible adhesion and encapsulation properties. When used on new, pristine, clean aluminum- it is believed to offer similar protection and adhesion to that of chemical conversion coatings. But there are some big caveats that if you speak to the experts- they will confirm. The first is- cleanliness. Epoxies must be applied to perfectly clean aluminum which means it has to be applied within minutes of cleaning. The second is that while epoxy adheres real well to most surfaces- only epoxy sticks well to epoxy once it's fully cured. Third- not all epoxies are the same. Some (like the Nason line) require an induction time which menas it must be mixed- allowed to stand for an hour to activate, then used within a certain window. This can make planning larger jobs troublesome. Lastly- many fillers are polyester based and polyesters do not chemically bond to epoxies. They can bond mechanically- which adds other problems- but the best system would involve epoxy fillers, which can be expensive and not as easy to work with. Once you have a process and system in place- it works well but there are many details and it's not as forgiving as the conversion process.

Now- you'll hear 1001 different opinions on processes that body shops use that works for them. Thankfully cars are more forgiving than many uses for painted aluminum and you can get away with less than ideal methods. I think most experts will agree that the best and most forgiving process is the chemical conversion route. Good epoxy over perfectly clean aluminum is a close second with some further process impacts. And there are probably dozens of other approaches that will work fine on your car. They may just not last as long as some others- but lots of things work for lots of folks -hence all the opinions.

entropy_engineering

This is my .02 so feel free to use as you see fit (or others correct me here if I'm wrong). I've have a bit of autobody experience (not a ton). A technical rep explained to me that you don't apply body filler to bare metal, as it can quickly collect moisture (which is bad being on metal of course). Use the etching primer, metal conversion, etc. then apply the filler. A decent site to nose around is autobody101.com. Also, if you fill after initial priming like I said, you will inevitably break through the primer in some spots. Don't spray an acid based primer over it again or it can attack the filler and cause long term bonding issues. Oh, I'm sure several will disagree, but I understand it to be a bad idea to wet sand body filler or high build primer because it can retain moisture that way and cause long term adhesion problems as well unless you have a heated booth at your disposal.

Richter12x2

Wow, I've painted a half dozen cars, including my RX7 with an aluminum hood and never knew it needed to be treated in a different way! It held up fine as long as I had it.

Mrmerlin

my 79 was also repainted and after 2 years with a piece of masking tape on the hood to mark a dent it pulled the paint coating off to bare metal.
Once this happened it was easy to use a blade to scrape the rest of the paint off the hood and fenders.
Turns out they used a regular dark primer and top coated it, nothing done to aluminum as as we could see
When I repainted the car it got a chemical wash then alodine then lite coat of zinc chromate then 2 sanded layers of epoxy primer then 2 base coats then 3 coats of clear, the paint was fantastic once it was sanded and buffed