Epoxy primer

I don't plan on painting my plane but want to apply epoxy primer to the wing spar tubes for corrosion protection. I'd hate to make the investment for the spray equipment and was wondering if it would be OK to brush the primer on? I'm using Poly fiber epoxy primer and the instructions only mention spraying.

Hi Jorge,
I brushed all my aluminum parts using the epoxy and it lays down ok - it's just not smooth and pretty - but gets a good coat on things. I also coated the inside of the spars by the "sloshing method" (see another thread about fabric covering).

Home Depot!

Get the foam brushes or the small little foam rollers and pans. This will be a little easier than a bristle brush to do the main areas.
You will probably need a small bristle brush to push the primer into the crevases.

If you are going through the extra work of coating your spars (probably 'cause you live near the coast in Florida) are you going to use a metal cleaner like Alumiprep 33?

Eric.

That would be my next question; what kind of prep do I need before primer?

Hi Jorge.

If I remember correctly, when I was building my Classic IV. What I read was do not scratch or avoid scratches in the aluminum tube, do not use a pencil on the aluminum tube for risk of creating a split. I believe I read this in the Skystar builders manual.
If this is the case, then do not attempt to use sandpaper.

You will have to clean it. Alumiprep 33 would be a good cleaner for this, you will have to rinse off with water and of course thorough dry before the application of epoxy primer.
If you don't want to do this then I suggest using MEK. Work on a small section at a time, very clean rags, wipe dry before the MEK can evaporate.

Eric.

Use a brush, and just slop the stuff on. That's what I did, you'll never
(hopefully) see the spar anyway, so it doesn't need to be pretty.

You might think about using zinc chromate inside the spar since you
do have some dissimilar metal with the SS rivets and the zinc chromate
will provide a sacrificial element rather than nearby aluminum. I don't
think polyfiber epoxy primer will provide that.

Regards,
Jeff

For sloshing the inside of the spar with a zinc chromate primer is a good idea, but I don't think you can purchase that these days. Everything has to be zinc chromate free (EPA and OSHA thing) So now it is chromate free:(
But the problem would be, the stainless steel rivets are pulled already. Tight up to the holes in the aluminum spar and I beam. Primer could never reach this area.
For those who have not inserted the I beam into the spar tube...Do not try priming the I beam or the inside of the spar. You will probably get it stuck on the way down the tube and that would be an expensive disaster.
But' on the other hand; nothing wrong with sloshing the inside. You probably cleaned the heck out of the tube before inserting the I beam, as clean as a rifle barrel by the time you were done. Now is the time to slosh if you feel that you need that protection. I didn't, so I never.

Maybe we can have a pole. How many builders sloshed the spars?

Eric.

You don't really need to worry about that so much. The key is that for
corrosion you need an anode, a cathode, and an electrolyte. With the
dissimilar metals you have two of the elements present. So the Aluminum
ends up feeding the SS Rivet. You can provide feedstock (so to speak)
for the rivets by putting some zinc chromate in there. So even if it is not
between the parts, it's presence on the outside should be enough. You
don't really need to scrub the parts that clean anyway. As long as they
aren't damaged from corrosion already the zinc chromate will still do
it's thing.

The biggest danger with Zinc Chromate is spraying it, and breathing the
resultant stuff in. Or grinding it into dust later (i.e. sanding), and breathing
that. Brushing, and sloshing are pretty safe.

Regards,
Jeff

I agree with what you are saying about the breathing in of paint particulate during spraying or sanding dust (from dry sanding and blowing the dust off)
I also understand that it is quite dangerous to produce at the manufacturer level because of the zinc chromate dust.

Also about the electrolyte, in this case there is probably damp salt air as the electrolyte. As you say anything could be better that nothing.

Cheers. Eric.