Help! Priming Aluminum Spars

[The Froon]

Man, this sucks - My back is still sore from bending over to chemically etch and prime the boom....

I started by sanding the boom down with 100 grit to remove oxidation, then applied West System's two-part aluminum prep system. This part was straight forward, no problem. Next I slightly thinned Interlux Pre-Kote with Interlux 333, and went on the apply the first coat of primer. I used a badger hair brush (no spray experience or equipment).

It appeared to lay on pretty good. I did notice some brush marks, but thought I would simply sand out, and then apply the consecutive coats. This was Thursday evening.



After leaving the office on Friday, I prepared to sand and apply the second coat. I did just that: I sanded the boom with 180 grit, blew off the surface, then applied Interlux 333 to wipe down the surface - was that bad? :) When I did this, the primer came off onto the rag.

[One Way David]

PreKote wiping off? Surely Interlux products are compatible. Not dry/cured? Contaminent in mixing container? Bummer.

Back. Your pics seem to show that you've run a line through the spar (love this nautical talk) and tied the ends to a chair. My back would not tolerate that either. I'd find a way to hang it a little higher so I didn't have to bend at all. Dave

[bcooke]

Here is a quote from Interlux's product instructions

Apply 1-2 coats. Depending on surface condition. Allow the necessary overcoating time between applications. Sand between primer coats with 120 grade (grit) paper until translucent. Remove sanding residue with a rag dampened with Brushing Liquid 333. Apply finish coats. Sand with 220 grade (grit) paper between Pre-Kote and finish enamel.

The critical part is the word 'dampened'. 333 breaks down the primer- that's why it is used as a thinner- use too much and it thins (and removes) the primer. Just damp enough to catch the dust, that's all you need, it will also soften the surface which will help the second coat adhere and bond with the first coat.

[The Froon]

I failed to mention that I live in Florida...and we have been having those soaker thunder storms in the afternoons. I think I pushed the project a bit by trying to sand and apply the second coat only 24 hours after the first. The air is holding a lot of moisture right now...

Should I sand the boom back down and etch the surface again, or can I just sand lightly and go with primer?

Thanks,

Brian

[bcooke]

Paint doesn't evaporate, it reacts with the oxygen in the air. Moisture will effect the process but not to the extent that you are thinking. (or at least what I think you are thinking...) The proper drying times are in the product instructions I just linked to.

Unless you sand down to bright shiny metal I don't see the need to re-etch. As the instructions say, sand to a translucent covering and then re-coat.

What kind of topcoat are you going with?

[Tim]

It's possible your problem could be related to whatever is beneath the primer: if the primer didn't bond correctly to the substrate, it would be more likely to come off with reducer after the fact. Can you elaborate on the initial steps before you applied the primer? The solvent really shouldn't remove the paint once its cured sufficiently to allow sanding, particularly since 333 is just kerosene and naptha.

You probably don't want to hear this, but at this point I think I'd remove all the primer and start from scratch. Depending on whatever the substrate is, you may even need to go back to raw aluminum and etching. I am not sure of the process you followed, so can't say for sure.

Getting paint to stick to aluminum requires meticulous prep and proper following of steps--this is why you see so many aluminum spars with paint peeling off and bubbling up--and with a setback like you've had I wouldn't risk trying to move on without getting back to square one. Sorry.

[The Froon]

Thanks guys. I used the West System aluminum prep kit - you can see this on their site. It has two parts, both require application and a rinse, time to dry between parts, and dry time before priming. It's possible that I pushed the process and did not allow full dry time. I used a scotch brite pad to scrub both part A and part B. I rinsed with water after each part application, and thought the surface was dry before proceeding.
I then thinned the Interlux Pre-Kote with 333 and brushed on. Thinking back, I noticed some parts of the primer 'gumming up' when sanding, so I am thinking that I did not allow full dry time.

Would anyone recommend an alternate etching system?

Also, if I have to fill some old rivet holes with thickened epoxy, should I do this before etching and priming or after I have primed? This will require using a counter sink bit to slightly over drill the hole to allow additional surface area for the epoxy to grab and adhere.

Thanks, thanks, thanks,

Brian

[Tim]

I am sure the West System etching materials are fine and don't suspect the products as long as the application was done properly. They have the same basic makeup as other products that do the same thing, like Alumiprep (phosphoric acid) and Alodine (chromic acid).

If your primer was gumming up during sanding, then it was not sufficiently cured, and this could easily lead to the problems you had with your wipe-down.

I am not clear on whether you should fill before or after doing your treatments. The best guidance I can give you comes from the Awlgrip application guide for aluminum, which indicates filling following etching and primer application; however, this application also is assuming the use of high-build epoxy-based primers, so this doesn't necessarily translate to the one-part Pre-Kote primer.

I didn't find any useful guidance for aluminum on the Interlux site.