RV-14 elevator skins primed too soon...need to scuff for trailing edge ribs

So I jumped the gun on priming the elevator skins. After priming the inside of the skins I read ahead in the plans and noticed that I was supposed to scuff the skins where the spars and trailing edge (foam) ribs are supposed to be bonded. I used the Akzo two-part epoxy primer. Should I just scuff the primer in those areas or try to remove it somehow? Or will the tank sealant from Vans stick to the primer well enough to hold?

I would remove it. If it's like the -10, I believe those ribs are structural and the primer is a big unknown here.

I've removed small amounts of the Akzo using a Dremel with a wire wheel. After that, scuff with sandpaper and clean per plans. The Akzo, if fully cured (24h+ after spraying IIRC), shouldn't be affected.

jacoby said:

I would remove it. If it's like the -10, I believe those ribs are structural and the primer is a big unknown here.

I've removed small amounts of the Akzo using a Dremel with a wire wheel. After that, scuff with sandpaper and clean per plans. The Akzo, if fully cured (24h+ after spraying IIRC), shouldn't be affected.

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Thanks for the reply!

Good luck. The -10 has this in the instructions:

"If priming the interior surface of the elevator skins, first mask off the locations of the trailing edge foam ribs so that they will be bonded to bare aluminum."

I would guess the -14 has similar verbiage. I would remove the primer.

Consideration

Not saying this is the correct answer but consider this. If the primer is that difficult for you to remove, why would it not hold if you simply scuff it and attach the ribs with pro-seal as directed. I would think the pro seal is more likely to Debond from the ribs before the primer debonds from the aluminum. Just thinking, but the decision is yours.

Paint removal

mulde35d said:

Not saying this is the correct answer but consider this. If the primer is that difficult for you to remove, why would it not hold if you simply scuff it and attach the ribs with pro-seal as directed. I would think the pro seal is more likely to Debond from the ribs before the primer debonds from the aluminum. Just thinking, but the decision is yours.

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I agree.
If you really want to remove it, try some tests on scrap. First try scuffing with a solvent like lacquer thinner, acetone or MEK. That should take it right off.

PM sent.

Dave

wirejock said:

I agree.
If you really want to remove it, try some tests on scrap. First try scuffing with a solvent like lacquer thinner, acetone or MEK. That should take it right off.

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I just tested this. Acetone will take off the dust and overspray. Maybe a slow removal of the regular primer (it colors the rag but isn't obviously loosening the primer; I can't tell if it is removing dust or actually bonded primer). It is not affected by isopropyl. I haven't tried MEK (it's just about impossible to find now and I only have a tiny bit left) or lacquer thinner. It reacts the same prepped with alumi-prep or bon-ami.

I'm actually a bit surprised it loosens a bit with acetone considering it's "fluid resistant". I have another 2k primer I use and it's unaffected by pretty much everything except phosphoric acid (even dot3 brake fluid doesn't touch it). All the parts I tested on were cured for 3+ days @ 72F.

It is probably ok to leave the primer in place. There is nothing in either TDS that mentions incompatibility.

That said, it's only about 10-15 min to remove the primer using a Dremel and the plans say to use scuffed bare metal. If something were to fail it is far more time and effort to fix.

mulde35d said:

Not saying this is the correct answer but consider this. If the primer is that difficult for you to remove, why would it not hold if you simply scuff it and attach the ribs with pro-seal as directed. I would think the pro seal is more likely to Debond from the ribs before the primer debonds from the aluminum. Just thinking, but the decision is yours.

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My current plan is to scuff it and see how much effort it takes to get to bare aluminum. If it proves to be too much I'll likely just scuff the primer and bond it. Thanks for the reply!

jacoby said:

That said, it's only about 10-15 min to remove the primer using a Dremel and the plans say to use scuffed bare metal. If something were to fail it is far more time and effort to fix.

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I agree. I'll test it and if I can get decent results without thinning the skin I'll probably suck it up and go that route. If not, I'll just scuff and bond. I have two elevator skins that are not primed so I'll follow the plans on those. If the elevator skins in question fail I guess I'll have my answer as to which method was more effective! Ha ha.