Sand metal skin before painting

[coolair]

I am preparing to paint my Savannah XL. The EAA recommends wet sanding the skin with 320 grit sand paper prior to priming.

 

Opinions?

 

If I do sand, how do I sand the area close to the rivets. In regular sanding the space around the rivets does not get sanded. Don't even mention individually sanding each rivet.

 

Tony

 

Denver, CO

 

 

[turboplanner]

I used a small sand blaster on the race car - compressor about 15-20 cfm, Grit grade to suit (not sure if they call it 320 or some other classification, but plenty of grades available), blast gun looks like a paint gun, and protection hood.

 

 

[Thruster87]

I am preparing to paint my Savannah XL. The EAA recommends wet sanding the skin with 320 grit sand paper prior to priming.Opinions?

 

If I do sand, how do I sand the area close to the rivets. In regular sanding the space around the rivets does not get sanded. Don't even mention individually sanding each rivet.

 

Tony

 

Denver, CO

Sanding is not a good idea as it can create stress risers and can damage rivet heads.Usually the process consists of acid washing with alumiprep 33 and scotchbrite pads then alodining and priming with a NON etch epoxy primer + top coat [before the primer hardens so no sanding required] or a water based surface preparation product like Prekote then priming with a NON etching epoxy prime + top coat.Self etch primers would remove the protective coatings.Always do a sample test piece to see everything is working the way it should. Cheers

[turboplanner]

Yes that's what I meant - disregard what I said, my aluminium skins weren't load bearing

 

 

[RetiredRacer]

Usually the process consists of acid washing with alumiprep 33 and scotchbrite pads then alodining and priming with a NON etch epoxy primer + top coat [before the primer hardens so no sanding required] or a water based surface preparation product like Prekote then priming with a NON etching epoxy prime + top coat.Self etch primers would remove the protective coatings.Always do a sample test piece to see everything is working the way it should. Cheers

Thruster, now you have me confused, and I would like to clarify it if I could.

 

I have just started to build a RV9a, and in the next week I'll be starting to do the first bit of priming. everything I have read so far (heaps) has stated Alodine (some have suggested Precote) and a etch primer (some mention acid washing with Alumiprep).

 

I have never used Alodine before so can you tell me what it actually does? and Does it actually take away the need for a etching type primer? And also what does the acid wash do?

 

I regularly work with aluminium and have found that unless I use a etch primer it is only going to peel, But with a etch primer you can unfold it, and then fold it again and it still does not peel.

 

Thanks

 

Bob Dennis

 

 

[coolair]

Thurster87

 

Thanks for your reply. What does "stress risers" mean? Why not use an etching primer?

 

Thanks

 

Tony

 

 

[Thruster87]

Stress risers are nicks,scratches.burrs,etc that a crack may develop from due to flexing [fatigue] of the aluminum structure. After precleaning with Alumiprep 33 or Metal Prep 79, rinse with water. Apply Alodine without dilution by brushing or immersion or swabbing with sponge. Rinse with water and surface is ready for paint. This is a visible coating which leaves the surface with a golden coloration.The alodized surface provides protection against weather and firmly bonds paint.ALODINE 1201 is a nonflammable, chromic acid based, coating chemical that will

 

produce a chrome conversion coating on aluminum and its alloys.

 

The coating formed by ALODINE 1201 is gold to tan in color and it becomes a

 

part of the aluminum surface. This chrome conversion coating offers the best

 

affordable substrate for both paint adhesion and corrosion resistance.If you use an etch primer it will remove the conversion coating. Just use an epoxy primer like Valspar VP50.If you do not want to do the pre-etching then use a etch primer directly on cleaned [metho] aluminum surface,but remember this will not have as good a long term resistance to corrosion .Cheer T87

 

 

[facthunter]

Alodine.

 

This is used on engine cases as well. The thickness of the coat is of no consequence to engine assembly and fit considerations so it is used on bores for journals and crankcase faces.. More complex chemicals are used on alloys containing magnesium, which corrodes very actively without protection. Overlapping sheet surfaces are treated prior to assembly(rivetting), if you want your aircraft to last. Good stuff. Nev.

 

 

[Thruster87]

This is used on engine cases as well. The thickness of the coat is of no consequence to engine assembly and fit considerations so it is used on bores for journals and crankcase faces.. More complex chemicals are used on alloys containing magnesium, which corrodes very actively without protection. Overlapping sheet surfaces are treated prior to assembly(rivetting), if you want your aircraft to last. Good stuff. Nev.

We use duralac for dissimilar metal contact and overlapping joints to minimize corrosion and try and keep it dry.Tank sealant can also be used for lap joints to keep the moisture out. Cheers T87

 

 

[coolair]

Thurster87

 

Thank you, thank you for the information. I did not want to sand anyway.

 

Aircraft Spruce prefers Metalprep 79 over Alumiprep 33. What's your preference? They both are the same price for me.

 

So you do not have to clean and degrease if you use Alumiprep?

 

Do folks ever leave the Alodine coating as the finished coat?

 

Tony

 

 

[RetiredRacer]

Thanks T87, that is the information I was needing. I'll be going with Alodine 1201 and an epoxy primer.

 

Thanks again

 

 

[Thruster87]

I use exclusively only Alumiprep 33 as it's the choice of the experts.It's what we used at Qantas so that's what I was familiar with. Cheers T87 [i did sand blast the barrels of a Yamaha DT250 when I was an apprentice and then alodined it and left it like that and it lasted as long as I had the bike which was about 4yrs.]:thumb_up:

 

 

[coolair]

T87

 

One more question.

 

Is it OK to get Alumiprep and Alodine on the black plastic parts?

 

Thanks,

 

Tony

 

 

[Thruster87]

T87One more question.

Is it OK to get Alumiprep and Alodine on the black plastic parts?

 

Thanks,

 

Tony

Not aware of any issues to date.As long as there are no circuit boards hiding under the plastic Cheers T87

 

 

[Yenn]

Be careful with Duralac. Its prime use is to protect bolts from corrosion, especially when used in wooden frames. It is considered seriously carcinogenic.

 

 

[Thruster87]

Be careful with Duralac. Its prime use is to protect bolts from corrosion, especially when used in wooden frames. It is considered seriously carcinogenic.

That was before my time now its' used for dissimilar metal contact and lap joints.Just about everything you use in the aircraft game has some sort of health warning on it Cheers T87:cool:

[Ultralights]

if the aluminium is Alclad, then sanding is a NO NO! Alclad is a layer of pure aluminium over the actual aluminium alloy. the layer of pure aluminium is a corrosion preventative.

 

 

[Guest Maj Millard]

T87 is spot on with his recommendations and suggestions. The acid etch and follow-up with Alodine is the way to go, on aluminum. An alodined finish is considered to be 'paint ready', after a good wipe down with a suitable prepsol, or other pre paint cleaner of course.

 

Both the acid etch and the Alodine are corrosive if left on, or migrate into other areas, so a good through wash off with clean water is required after the applications.

 

Many alum boat builders wonder why their expensive 2 pack paint hasn't stuck to their nice clean Aluminim. If they acid etch and alodine, it will .........................

 

 

[shafto]

EAA recommends wet sanding the skin with 320 grit sand paper prior to priming.

Why would EAA recommend this? That just is insanity! Like Thruster87 said it will just leave stress risers, and totally destroy the alclad!

 

When you do use the Alodine, read the MSDS and stick to its saftey warnings. Alodine is not something you want in your eye let alone on your skin, so make sure your covered up

 

 

[coolair]

T87

 

I am ready to start painting my Savannah XL. I am using your recommendation of washing with Alumiprep and coating with Alodine. I used one part Alumiprep to three parts water, applied with a brush and scrubbed with a sponge; wait three minutes then wash with clean water for five minutes; apply non-diluted Alodine with a brush; wait three minutes then power wash with clean water for five minutes then dry with a cotton cloth. The result is a mottled/streaked golden finish. Is it supposed to look good at this point (it do not)? It will be covered with primer and paint. I will try to add photos of the result to the photo gallery.

 

Is this procedure acceptable?

 

Should I make the Alumiprep more concentrated?

 

Thanks

 

Coolair