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Author Topic: Priming bare aluminum questions  (Read 2364 times)
Iceni John
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« on: August 04, 2013, 05:52:54 PM »

I'm now in the middle of stripping my aluminum roof down to bare metal  -  some of the original paint was flaking off, so I'm not taking any chances with the remainder that may or may not be secure.   (Talk about tedious  - on a step ladder or on my knees on the roof for hours in the hot sun, scraping it off with a hot air gun inch by inch.   I cannot use chemical strippers or blasting, because I'm doing this in the storage yard next to other RVs.   Oh yes, don't forget the millions of rivets also there.   I think it's character-building . . .)

I have some questions about prepping and priming.   After reading various threads here and elsewhere I'm more confused than ever!   Ideally I would use Metalprep 79 to etch the bare aluminum, then maybe Alodine, then prime and top coat, but phosphoric acid etchers need to be washed off after a few minutes  -  I cannot wash off the acid because the sides of the bus won't be stripped to bare metal and I don't want acid running down the good paint there.   It's the same story with Alodine  -  it also has to be washed off after application.   If I use a self-etching primer it should work, but Eastwood's isn't available for sale in California in gallons, only in quarts and aerosols;  Rustoleum is available everywhere but is suspiciously cheap compared to SEM, but again they are only available in spray cans, not much use for doing 350 sq.ft.

If I thoroughly sand or wire-brush the aluminum to give it a rough surface, will that be enough for the eventual Rustoleum Professional primer and enamel that I'll roll on, or do I absolutely have to etch it first?   Are there any other self-etching primers available in CA by the gallon that I can roll or brush on?   Senik Paint here wasn't much help for me, and I can't tell if Aircraft Spruce has anything suitable.   Whatever I do has to be the last time I'll ever paint this darn roof!   I just don't seem to have many options for my particular circumstances.

Thanks, John
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1990 Crown 2R-40N-552:  6V92TAC, DDEC II, HT740, Jake.      Hecho en Chino.     
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Utahclaimjumper
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« Reply #1 on: August 04, 2013, 06:14:36 PM »

 Etching and alodine is really old school and labor intensive ,please don't use a "WIRE" brush on aluminum, keep steel away from alloy. Strip as needed to remove any loose paint, you should be able to use the stripper and scrape method even close to other RVs if reasonably careful.. Prep for paint using a self etching urethane primer, follow with a urethane top coat of your choice, don't use the "base coat clear coat" method on a roof application, the clear will peel in short order on a roof.>>>Dan (30 year A/P-IA)
« Last Edit: August 05, 2013, 10:47:59 AM by Utahclaimjumper » Logged

Utahclaimjumper 
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Iceni John
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« Reply #2 on: August 04, 2013, 07:49:21 PM »

please don't use a "WIRE" brush on aluminum, keep steel away from alloy. .>>>Dan (30 year A/P-IA)
I'm using some brass wire wheels that work pretty well to get paint off the million rivet heads, so no worries there about steel not playing well with aluminum.

Can I roll those urethane primer and top coats on, or do they have to be sprayed?   Rolling is my only practical way to do it  -  I don't think spraying is kosher in a storage yard!   Appearance is not a priority  -  if it looks good at 50 feet or 50 MPH I'll be chuffed, and anyway the roof's going to have solar panels and a catwalk over most of it when I'm done.   I'll also mix the Hy-Tech ceramic additive into the top coat(s) for another few degrees of insulation.

Thanks, John
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1990 Crown 2R-40N-552:  6V92TAC, DDEC II, HT740, Jake.      Hecho en Chino.     
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TomC
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« Reply #3 on: August 04, 2013, 09:27:09 PM »

Here in California, we have limited selection of primers. A couple years ago I replaced one of my panels on the bus (had a close encounter). After sanding the bare aluminum with 320, used a water based primer that I rolled on. Then used regular sanding primer over that, then Dupont 5000 single stage urethane paint. So far so good. Good Luck, TomC
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« Reply #4 on: August 05, 2013, 10:51:54 AM »

 In your situation I would say the "roll & tip" method would work well, urethane does not care, I would thin it more for that method than for spray.>>>Dan
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Utahclaimjumper 
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Iceni John
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« Reply #5 on: August 05, 2013, 12:36:02 PM »

Are there any readily-available brands of urethane primer and topcoat you would recommend?   Bear in mind that California outlaws some of the good stuff available elsewhere, and that I would prefer to buy a brand that's available from multiple suppliers.   Rustoleum makes no mention that their Professional line is urethane, and Sherwin-Williams urethane enamel does not seem to have a matching self-etching primer.   How important is it to match primer and top coat, for example could I use Rustoleum self-etching primer with S-W urethane topcoat, or would bad things happen?   When I asked the local S-W store about what I'm needing I just got frog-like expressions from the sales dudes there  -  I guess I need to know exactly what I want if I order something from them!

Sorry for all these questions  -  I'm a complete newbie when it comes to paint, so I will happily take the advice of those who know.

Many thanks, John
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1990 Crown 2R-40N-552:  6V92TAC, DDEC II, HT740, Jake.      Hecho en Chino.     
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« Reply #6 on: August 05, 2013, 04:23:52 PM »

 Go to an auto body paint supplier, they are the real deal.>>>Dan (But when buying, use commercial type urethane, not the automotive (more expensive)type. The only difference is color choices, sometimes called "fleet colors"
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Utahclaimjumper 
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« Reply #7 on: August 05, 2013, 04:45:10 PM »

Martin/Senour has a good "Green " paint system that is approved for the holy land of CA  a plus is it is easy to use for the beginner
« Last Edit: August 05, 2013, 05:03:37 PM by luvrbus » Logged

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« Reply #8 on: August 06, 2013, 11:12:17 AM »

Use Alumaprep on it. Seriously. Go to a PPG store and ask for an etching primer for the aluminum. We did and our paint is bomb proof.
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Scott & Heather
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« Reply #9 on: October 06, 2013, 07:58:18 PM »

Sorry to keep asking newbie questions about this, but I found something yesterday that may work well, and I just want to hear the collective wisdom of the paint-savvy folk here before I start:

McFadden-Dale Industrial Hardware in Santa Ana CA (now that's a real nuts 'n bolts store, and I'm there every week buying everything I need) has gallon cans of Mar-Hyde 5113 self-etching pre-treatment primer.   It must be used together with a normal primer  -  you can't topcoat directly over it.   My plan is to sand and wire-brush the whole roof (it's now completely stripped down to bare aluminum), wipe it with acetone and tack cloths, roll on the Mar-Hyde, then roll on Rustoleum Professional 242259 oil-modified alkyd clean metal primer that's also in stock there.   I'll then roll on two coats of Rustoleum Professional 242256 oil-based white alkyd enamel with the Hy-Tech ceramic insulation added into it, and a possible thin third coat without ceramic to smooth the final surface.   I had thought of using Rustoleum's 8781402 aluminum primer straight on the metal, but nobody here has it or can get it.   Using the Mar-Hyde is more expensive, but if it helps the paint's adhesion even slightly then I'll do it  -  there's no darn way I ever want to repaint my roof again!

Mar-Hyde's info doesn't say anything about rolling it on (it's intended for spraying, but I can't do that in the RV yard)  -  I see no reason I can't roll it if it's a cool day.   It only needs a thin coat, then the Rustoleum primer will be higher-build.   Can anyone think of any reason this won't work  -  will the Rustoleum primer be OK on top of the Mar-Hyde?   Are there any possible chemical incompatibilities that could cause bubbling or peeling later?

Thanks, John      
« Last Edit: October 06, 2013, 08:11:36 PM by Iceni John » Logged

1990 Crown 2R-40N-552:  6V92TAC, DDEC II, HT740, Jake.      Hecho en Chino.     
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RJ
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« Reply #10 on: October 07, 2013, 12:54:56 AM »

John -

Clifford (lvrbus) is just across the Colorado River and slightly north of Needles (I-40) in AZ.  Wonder if you could shuttle your coach over there and get some of the "good stuff" the Californication Environmental Wackos won't let us have. . .  Just thinkin'

FWIW & HTH. . .

 Wink
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RJ Long
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Iceni John
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« Reply #11 on: October 07, 2013, 08:14:29 AM »

John -

Clifford (lvrbus) is just across the Colorado River and slightly north of Needles (I-40) in AZ.  Wonder if you could shuttle your coach over there and get some of the "good stuff" the Californication Environmental Wackos won't let us have. . .  Just thinkin'

FWIW & HTH. . .

 Wink
Yup, that thought had ocurred to me, making a quick jaunt to AZ or NV or even MX to get the roof repainted.   However, at this point I've done the worst of it, with only the easy(ish) painting part left to do.   I just slightly neurotic about making sure I've done the best possible prepping job before I start the actiual painting, and that's maybe something where "professional" painters (at least, those I could afford!) could possibly take shortcuts if I wasn't scrutinizing their every move.

I want to get this darn roof done by the end of this month, then I can begin the fun stuff building the roof walkway and support frames for my solar panels.   Maybe I'll be so paint-focused that I'll instead end up repainting the whole bus?   My friends have been bugging me for so long about "that big yellow bus of yours", so a change of color is long-overdue!

John
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« Reply #12 on: October 07, 2013, 10:24:04 AM »

Spi epoxy, after sanding with 80 grit paper. use the metal prep wash, not acetone. you can paint over it within 7 days without sanding. Here is a link to the site. check the forum for other good advice. the epoxy adheres better than anything, and the next coat bonds chemically. They also sell single stage paint in white, red, or black. This is the BEST paint you can buy, at the best price. http://www.southernpolyurethanes.com/
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« Reply #13 on: October 07, 2013, 07:47:00 PM »

After reading this thread I am wondering if you have thought about what etching means. To me it means creating small rough areas that the paint can grip to. Have you thought about roughing it up with Scotch Bright then maybe priming with Zinc Oxide. Zinc Oxide is available from Aircraft Spruce and covers very well.
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« Reply #14 on: October 07, 2013, 08:09:36 PM »

My search gets zinc chromate as the primer, not zinc oxide.
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Charles in SC
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« Reply #15 on: October 08, 2013, 07:21:16 PM »

In my Aircraft Spruce catalog Zinc oxide primer is on page 374. Part # 09-00962 for a quart. 09-00960 for a gallon in green.
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Larry B
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« Reply #16 on: October 11, 2013, 07:21:55 PM »

I am not a professional paint, but I did paint my own bus with air sprayer and a brand name paint called Endura. It can be mixed in any color you want. It is the same paint used on industrial equipement ( drilling rigs etc.- nothing takes more of a beating than a drilling rig). This is tuff paint and cheaper than auto. paint. This may not be a desirable option to you but most of all I wanted to give you a heads up on aluminum. Aluminum starts to oxidize in about four hours, so from the time you clean the bare aluminum until you get some paint on it should not be any more than four hours. I was told this by two separate body  shops when asking questions same as you are doing now. If you are doing this job by yourself, this will not be possible, but you do not have to paint the whole roof in same day and it does not sound as you are looking for perfection. For cleaning the bare aluminum I used lacquer cleaner. same stuff you clean your spray gun with. I put the cleaner in an old spray bottle (windex) and sprayed the cleaner right on the bare metal. With a rag keep on wiping and reapply cleaner until no more black oily residue is evident. Aluminum is oily be nature of the metal.You are ready to paint. Hope this helps a bit and I am not too late before you paint.
           Larry B
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Iceni John
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« Reply #17 on: October 11, 2013, 09:32:45 PM »

Thanks, Larry, for that good advice.   I'll start tomorrow.   Fortunately the roof is in seven sections, each about 5 feet wide, so I can do this in stages.   I'll first sand with 100-grit aluminum oxide paper to roughen the surface and remove the oxide and corrosion on it, and wire-brush (with brass wire wheels) around the rivets where I can't sand, then wipe down with PreClean 191 and microfiber cloths.   Without any delay I'll brush on two thin coats of the Mar-Hyde etching pre-treatment primer, then as soon as it's dried I'll roll on the Rustoleum clean-metal primer over the Mar-Hyde.   Yes, I know that aluminum oxidizes immediately after it's been sanded, thus the hurry to get the etching primer on as soon as possible.   I opened the can of Mar-Hyde this evening to have a look at it  -  boy, does it smell!   It's way too volatile to pour into a paint tray and roll on, so I'll have to brush it instead.

If I can get one section primed in a day I'll be happy.   At that rate it will take me a solid week to prime the whole roof, then I have two or three coats of the white top coat to roll on.   I intend to get it all done by the end of the month, before the first winter rains come.

Charles, yes, I know about the importance of keying the surface for the paint to adhere well.   My thought is that both sanding and using an etch primer should work as well as I can do in the circumstances (outdoors, with expensive RVs either side of me).   Some of the original paint had separated off the roof, presumably from careless or inadequate preparation, hence my desire to not take shortcuts this time.

Wish me luck tomorrow!
John
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« Reply #18 on: October 13, 2013, 08:12:19 AM »

doing this in sections makes the most sense. it will eliminate dry edges where adhesion could fail. you need to use an etching primer with phosphoric acid or zinc chromate. there are tons of them out there. to my knowledge urethanes etching primers will need to be sanded before color coating. you do not want to have to sand all that surface area. be sure to wear plastic gloves when stripping to keep oils and sweat off the surface. I always finish the sanding with 80 grit paper on a da sander.  it gives you a little extra physical grip along with the chemical grip. an 80 scratch on bare metal is not the same as an 80 grit scratch in paint. avoid polishing the surface with lighter grits. you can wash with a metal prep before etch priming. don't prime in the sun or heat. you don't want the solvent(acid) flashing off before it etches into the top surface. etching primers can be topcoated directly over the etch primer at the prescribed interval. waiting too long will have an adverse effect on paint adhesion. too soon may form solvent popping thru the paint surface. if you pass the interval for topcoating just re etch over top of your existing etched surface. it will bite right back in. remember the primed surface is not a barrier against weather. it needs to be top coated for moisture protection. also best to spray on low humidity days. don't cheat with top coats. at least 3 good medium coats of a urethane paint--no acylic enamels(they craze over time). insufficient paint millage can lead to delamination (peeling)of the paint--esp on a top surface. make sure you get the tech fact sheets with all the products you are buying. the counter guy may not be up to speed on every aspect of every product. the sheets will have everything you need to know and the retailer has them or he can print them online. good luck
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