Van's Air Force
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metal prep question?
- Thread starter miyu1975
- Start date
Prepping Aluminum
Both Metal Prep 79 and Alumiprep are acid washes designed to etch the surface of the aluminum. Both are diluted with water prior to use and are rinsed after application. [Note that the Spruce catalog says that Metal Prep 79 is wiped off...however, the MSDS says to rinse].
Both are normally followed by a chromate treatment process (such as Alodine).
Prekote is a newer process and is supposed to be a more environmentally friendly process: no chromates, acids, or solvents. I haven't used it so can't comment. Having said that, I have used Alumiprep, followed by Alodine, followed by Akzo epoxy primer. I've also experimented with:
Dawn dishwashing detergent rinse, followed by Akzo primer.
Coleman fuel cleaning, followed by Akzo primer.
MEK cleaning, followed by Akzo primer.
I all cases I scuffed the aluminum well and rinsed/wiped until the rags weren't picking up any oxidation.
Results: Akzo primer sticks well, regardless of method listed above. My opinion is that an etching/chromating process might be required/desired in a maritime climate, but isn't necessary for primer to stick well as long as the aluminum is scuffed to provide some "bite" and is cleaned well/has no oils/residues on the surface.
I've also had good luck with other self-etching primers, such as NAPA 7220.
YMMV.
Mike
Both Metal Prep 79 and Alumiprep are acid washes designed to etch the surface of the aluminum. Both are diluted with water prior to use and are rinsed after application. [Note that the Spruce catalog says that Metal Prep 79 is wiped off...however, the MSDS says to rinse].
Both are normally followed by a chromate treatment process (such as Alodine).
Prekote is a newer process and is supposed to be a more environmentally friendly process: no chromates, acids, or solvents. I haven't used it so can't comment. Having said that, I have used Alumiprep, followed by Alodine, followed by Akzo epoxy primer. I've also experimented with:
Dawn dishwashing detergent rinse, followed by Akzo primer.
Coleman fuel cleaning, followed by Akzo primer.
MEK cleaning, followed by Akzo primer.
I all cases I scuffed the aluminum well and rinsed/wiped until the rags weren't picking up any oxidation.
Results: Akzo primer sticks well, regardless of method listed above. My opinion is that an etching/chromating process might be required/desired in a maritime climate, but isn't necessary for primer to stick well as long as the aluminum is scuffed to provide some "bite" and is cleaned well/has no oils/residues on the surface.
I've also had good luck with other self-etching primers, such as NAPA 7220.
YMMV.
Mike
TerryWighs
Well Known Member
Ryan,
I am doing the same process. Using AlumiPrep/Alodine preparation, followed by AKCO epoxy primer, then painting with Acy-Glo.
Using "Thunderstorm Grey", and I am very happy with the results. Should be extremely durable...Some samples of the process are on my building site. I have been painting interior parts separately as I go...
I am doing the same process. Using AlumiPrep/Alodine preparation, followed by AKCO epoxy primer, then painting with Acy-Glo.
Using "Thunderstorm Grey", and I am very happy with the results. Should be extremely durable...Some samples of the process are on my building site. I have been painting interior parts separately as I go...
BSwayze
Well Known Member
Ryan,
I've used Metal Prep 79 as a metal cleaner/etcher. It's a great product. I've also used Coleman lantern fuel and MEK with equally good results. I don't have any experience with Alumiprep, but I really like the Metal Prep product. I scuffed the surfaces with scotchbrite and the Metal Prep solution, rinsed, then primed when dry. I've never used any alodine product. My primer bonds very well.
The only question I can't answer is how you're going to rinse everything, since your fuselage is already built. I cleaned and primed before riveting. Let us know how it goes.
I've used Metal Prep 79 as a metal cleaner/etcher. It's a great product. I've also used Coleman lantern fuel and MEK with equally good results. I don't have any experience with Alumiprep, but I really like the Metal Prep product. I scuffed the surfaces with scotchbrite and the Metal Prep solution, rinsed, then primed when dry. I've never used any alodine product. My primer bonds very well.
The only question I can't answer is how you're going to rinse everything, since your fuselage is already built. I cleaned and primed before riveting. Let us know how it goes.
miyu1975
Well Known Member
thanks everyone...still up in the air as to what I intend. I am thinking I will be using the maroon scotch bright, soap and water, acetone, lint free wipe down clothe, and epoxy primer, then paint....this for the interior. Exterior...still a ways away for that decision.
terrykohler
Well Known Member
You Might Think About Skipping the Scotchbrite
Ryan:
Mechanically scuffing a surface before using an etching primer is a little like using a belt and suspenders to hold up your pants. The purpose of the etchant is to "attack" the surface in preparation for paint. Under a microscope, an etched surface will appear rough. Chemical conversions such as alodine provide additional corrosion resistance as well as a good base for paint adhesion. Scuffing an alclad surface is in most cases detrimental, as the purpose of the clad or pure layer is to provide additional corrosion resistance. Even if the scuffing you're doing doesn't remove the alcald, you're very likely to have scratched thru in enough places to render the coating ineffective. Proper etching won't do that. In any event, a good paint job will make up for any damage you may cause. I guess from my point of view, why add the additional labor.
Terry, CFI
RV-9A N323TP
Ryan:
Mechanically scuffing a surface before using an etching primer is a little like using a belt and suspenders to hold up your pants. The purpose of the etchant is to "attack" the surface in preparation for paint. Under a microscope, an etched surface will appear rough. Chemical conversions such as alodine provide additional corrosion resistance as well as a good base for paint adhesion. Scuffing an alclad surface is in most cases detrimental, as the purpose of the clad or pure layer is to provide additional corrosion resistance. Even if the scuffing you're doing doesn't remove the alcald, you're very likely to have scratched thru in enough places to render the coating ineffective. Proper etching won't do that. In any event, a good paint job will make up for any damage you may cause. I guess from my point of view, why add the additional labor.
Terry, CFI
RV-9A N323TP
terrykohler
Well Known Member
Surface Needs to be Prepped
Ryan:
The proper surface "texture" can be achieved by etching mechanically (with scotchbrite or aluminum oxide pads) or chemically (an etching process usually followed by a conversion). Doing both is ... redundant. Pick one, as paint has a difficult time sticking to a smooth, rolled aluminum surface. I'm not a painter (retired metallurgist), but to me etching makes more sense for reasons already mentioned. Why use alclad if you're just going to end up grinding it off? Not sure about PreKote, but I believe the process does not involve acids, but does involve mechanical etching.
Terry
Ryan:
The proper surface "texture" can be achieved by etching mechanically (with scotchbrite or aluminum oxide pads) or chemically (an etching process usually followed by a conversion). Doing both is ... redundant. Pick one, as paint has a difficult time sticking to a smooth, rolled aluminum surface. I'm not a painter (retired metallurgist), but to me etching makes more sense for reasons already mentioned. Why use alclad if you're just going to end up grinding it off? Not sure about PreKote, but I believe the process does not involve acids, but does involve mechanical etching.
Terry
rzbill
Well Known Member
Cutting through alclad caps - not
In the thicknesses we use, the pure aluminum alclad caps are 2.5% of total thickness per side which works out to between 0.0005 to .0010" per side depending on the sheet you have in your hand.
Surface roughness is commonly measured in microinches or 0.000001" units. For instance, our new sheet is probably on the order of 2-8 microinch (going by visual reflectivity experience, not measured) and the nicely scuffed surface is going to be around 32-64 microinches, probably closer to 32. 125 is very coarse and probably not achievable with the grits normally in the aviation toolbox.
125 microinches is 0.000125". That is not even close to breaking the cap.
So, sure you can break the pure caps, but you have to go at it like you WANT to break thiem.
Does anybody remember the appearance change when tapering the top wing skin corner on a 6/7/8? And how hard it was to get that far? The appearance change was when you hit the copper infused 2024.
In the thicknesses we use, the pure aluminum alclad caps are 2.5% of total thickness per side which works out to between 0.0005 to .0010" per side depending on the sheet you have in your hand.
Surface roughness is commonly measured in microinches or 0.000001" units. For instance, our new sheet is probably on the order of 2-8 microinch (going by visual reflectivity experience, not measured) and the nicely scuffed surface is going to be around 32-64 microinches, probably closer to 32. 125 is very coarse and probably not achievable with the grits normally in the aviation toolbox.
125 microinches is 0.000125". That is not even close to breaking the cap.
So, sure you can break the pure caps, but you have to go at it like you WANT to break thiem.
Does anybody remember the appearance change when tapering the top wing skin corner on a 6/7/8? And how hard it was to get that far? The appearance change was when you hit the copper infused 2024.
wirejock
Well Known Member
I did a bit of research on this. I'm no expert but here's what I learned.
Aluminum Oxide is the enemy. It's a very thin surface the forms on aluminum. On steel we call it rust. It forms very quickly and it's hydophobic. Nothing sticks well to it.
Try pouring water on your untreated part. You'll see it doesn't coat the surface.
Since it's thin, it is easy to remove but forms again within an hour or two.
Removal can be done chemically or mechanically. Sol ents are not as good at removing the aluminum oxide layer. Chemical acids work well but not so good for you or the environment. Scotchbrite works well and also provides a mechanical bond but you still have to clean.
I found a trick the works for both. Dry powdered cleansers like Comet and Ajax combined with scotchbrite work very well. Dust the pad, wet, scrub and rinse. The rinsed parts are hydrophillic. Wet the part and water coats the surface. The best part is it's ok for your hands and the environment.
Paint adhesion tests seem to show the process is as good or better than solvents.
Aluminum Oxide is the enemy. It's a very thin surface the forms on aluminum. On steel we call it rust. It forms very quickly and it's hydophobic. Nothing sticks well to it.
Try pouring water on your untreated part. You'll see it doesn't coat the surface.
Since it's thin, it is easy to remove but forms again within an hour or two.
Removal can be done chemically or mechanically. Sol ents are not as good at removing the aluminum oxide layer. Chemical acids work well but not so good for you or the environment. Scotchbrite works well and also provides a mechanical bond but you still have to clean.
I found a trick the works for both. Dry powdered cleansers like Comet and Ajax combined with scotchbrite work very well. Dust the pad, wet, scrub and rinse. The rinsed parts are hydrophillic. Wet the part and water coats the surface. The best part is it's ok for your hands and the environment.
Paint adhesion tests seem to show the process is as good or better than solvents.
XOverZero
Well Known Member
No chlorine
If I may pop in with (arguably) 2 cents worth...my preference in household cleansers for airplane parts is Bon Ami rather than Comet or Ajax. It doesn't contain chlorine, cleans very well, and rinses off completely. This is what we used in manufacturing aircraft radomes to achieve a water-break free surface prior to a secondary bond operation.
If I may pop in with (arguably) 2 cents worth...my preference in household cleansers for airplane parts is Bon Ami rather than Comet or Ajax. It doesn't contain chlorine, cleans very well, and rinses off completely. This is what we used in manufacturing aircraft radomes to achieve a water-break free surface prior to a secondary bond operation.
The part that you show in that picture is steel, not aluminum. Unless you have access to cad plating equipment I would just mechanically remove any rust/oxidation using scotchbrite, sandpaper or wire brush, then clean with acetone and epoxy prime.
wirejock
Well Known Member
rv7charlie
Well Known Member
speaking of extra steps...
A fellow RVer near the east coast of FL made test tabs of 2024 alclad. Bare (unprocessed), etched/alodined, epoxy primed, alodine/epoxy primed, etc. Wedged them all in slots in a 2x4 & stuck them in a 5 gal bucket of sea water for months, pulling them out every few days to inspect. The etch/alodine tab survived just as well as the epoxy primed & other 'exotic' processes. The only tab that didn't fare well was the unprocessed piece.
I doubt you can measure the weight difference of an alodined part without lab gear. Pretty sure it will be easy to measure the weight difference with epoxy or any other coating. Very confident about the weight difference of the wallet....
For cleaning of small to medium size parts, up through the main ribs, IIRC, I had good luck with our dish washer & regular dishwasher detergent for the 1st cleaning step prior to etch/alodine. Cleans/degreases very well, and the only labor was carrying the parts to/from the house. (Not my idea; like most good ones, someone with more experience told me about it.)
Charlie
A fellow RVer near the east coast of FL made test tabs of 2024 alclad. Bare (unprocessed), etched/alodined, epoxy primed, alodine/epoxy primed, etc. Wedged them all in slots in a 2x4 & stuck them in a 5 gal bucket of sea water for months, pulling them out every few days to inspect. The etch/alodine tab survived just as well as the epoxy primed & other 'exotic' processes. The only tab that didn't fare well was the unprocessed piece.
I doubt you can measure the weight difference of an alodined part without lab gear. Pretty sure it will be easy to measure the weight difference with epoxy or any other coating. Very confident about the weight difference of the wallet....
For cleaning of small to medium size parts, up through the main ribs, IIRC, I had good luck with our dish washer & regular dishwasher detergent for the 1st cleaning step prior to etch/alodine. Cleans/degreases very well, and the only labor was carrying the parts to/from the house. (Not my idea; like most good ones, someone with more experience told me about it.)
Charlie
Raymo
Well Known Member
My 2 cents:
If you want to prep for paint but are not ready to paint for a while:
Scuff/acid etch and alodine to protect the surface from corrosion. Prime when ready some time down the road.
If ready to prime:
Scuff/acid etch and prime as soon as soon as possible and within the 2 hour window mentioned by others.
Edit:
Also use alodine for areas that need good grounding, like antenna mount locations.
If you want to prep for paint but are not ready to paint for a while:
Scuff/acid etch and alodine to protect the surface from corrosion. Prime when ready some time down the road.
If ready to prime:
Scuff/acid etch and prime as soon as soon as possible and within the 2 hour window mentioned by others.
Edit:
Also use alodine for areas that need good grounding, like antenna mount locations.
Last edited:
Should this be a touch free spray of acetone or should I use a lint free cloth and wipe.
Raymo
Well Known Member
The latter. You are removing the acetone/solvent along with any contaminates.
