Scuffing prior to priming

With all the pieces to prime in the fuselage I am trying to do more time efficient prep work. On the empanage I scuffed the parts with the 3M scotch brite pad almost immediately before primming as I didn't want to let the bare aluminum to be exposed for a long time. However, what is the general consensus regarding scuffing prior to priming without running into an oxidation issue? Can I do it the night before?

Before priming I always wipe down with acetone wearing nitrile gloves to prevent the transference of any oils / contaminants. Priming with azko.

If you wipe it down again, prep the night before works.
Use this with your Scotch Bite pads: https://www.aircraftspruce.com/catalog/cspages/alumiprep.php

Carl

Mark,

The aluminium oxidises in a fraction of a second !

That is the smell you can sense when you use Scotchbrite. Pure aluminium is an unusual material in as much as it oxidises and then just about stops - it protects itself which is why it makes such a good material when alloyed with other metals and flashed over with pure aluminium.

That said, by Scotching and then priming you prevent any contamination by oil, grease and dirt. Yes, it can be done a few hours before. Try get some panel wipe from a local auto paint supplier - it is much less aggressive and user friendly than acetone. It also has anti silicone additives which all help. It is a combination of slower evaporating solvents and gets the surface really clean.

so just to muddy the water i would think the solvent from auto paint suppliers would not address the needs of painting aluminum as there is not much on a car. i thought the end all to prepping aluminum for paint was to clean it with something like comet, ajax, etc. and rinse. having tried it it certainly does decrease surface tension which is an indication the surface is more attracted to a liquid.
is there more to it or other considerations [besides degreasing]?

Depends what type of primer you intend to use.

Chromate primers (not Alodine, primer) convert the surface - thin film.

Non chromate acid etch primers use acid as the key and a thicker film to protect.

Epoxy have no real etch and rely on a surface key treatment - thickest film thickness.

The panel wipe is primarily a degreaser and anti silicone prep, the Scotch does the main job. I agree, using other products will etch the surface and prep it, but let's use Cessna, Piper and Beech as a base line - they never did the amount of work on production aeroplanes that we tend to do.

As with anything you are creating, you balance your time, budget and mission to suit how you build. Just keep a sensible eye on the basics as you progress.

mike newall said:

Depends what type of primer you intend to use.

Chromate primers (not Alodine, primer) convert the surface - thin film.

Non chromate acid etch primers use acid as the key and a thicker film to protect.

Epoxy have no real etch and rely on a surface key treatment - thickest film thickness.

.....

Click to expand...

Aren't several of the primers w use a combination of epoxy and chromate?

The Akzo 463-12-8 epoxy primer has strontium chromate as a major ingredient.

Perhaps we can get the best of two classes of product combined...

I can only speak from experience but I used the Prekote product and a scotch brite pad to prep my empennage pieces. Within 10-15 minutes I sprayed with AKZO and had great results. It's a bit time consuming using a spray gun and I wasted a bit trying to gauge how much I needed but in the end for my first run with using a 2 part paint and a spray gun was a good time.

For the fuselage and wings I went with the QB so decided for ease I would try just some rattle can stuff. I prepped the same way but then just rattle canned stuff on. In hind sight if I had to do it again I would of stuck with the AKZO stuff. I've gone through a good 20 cans of rattle paint and it scratches a lot easier than AKZO.

Aluminum Oxide

bobnoffs said:

so just to muddy the water i would think the solvent from auto paint suppliers would not address the needs of painting aluminum as there is not much on a car. i thought the end all to prepping aluminum for paint was to clean it with something like comet, ajax, etc. and rinse. having tried it it certainly does decrease surface tension which is an indication the surface is more attracted to a liquid.
is there more to it or other considerations [besides degreasing]?

Click to expand...

You are mostly correct but the goal is to abrade for mechanical adhesion then remove the aluminum oxide layer as close to applying primer as possible. The AO layer is only a few microns thick but it's a hydrophobic surface. Paint doesn't stick very well. You want a hydrophilic surface. As you mentioned, water will stick.
Scotchbrite abrades very well and creates a good surface for mechanical adhesion. A light scuff is plenty. No need to grind away.
The AO layer is very thin. Comet and Ajax work well with gray scotchbrite to remove the AO but they contain Sodium Hypochlorite (bleach) which is not so good for aluminum. Try BonAmi. No bleach. You are correct. The water should coat the surface not bead up. Rinse well, dry and prime as soon as possible. AO reforms quickly.

az_gila said:

Aren't several of the primers w use a combination of epoxy and chromate?

The Akzo 463-12-8 epoxy primer has strontium chromate as a major ingredient.

Perhaps we can get the best of two classes of product combined...

Click to expand...

You are correct !

The one I was looking at in the UK had no chromate in. Personally, there are enough chemicals trying to kill us in our build that I avoid chromates at all costs. Other non chromate primers offer sufficient protection for my needs.