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jferraro17 01-29-2010 01:14 PM

Priming 2 different materials
So, I've reached the point with the emp tips that I'm happy with the finish--at least I think I am.

I would like to coat these areas with primer since a final paint job is years down the road. Any suggestions on what to use? My concern is, the combination of fiberglass and aluminum in the areas that need to be coated. I'm pretty sure I can't use the self-etching rattle cans I've been using throughout the build for parts.

Thanks for any advice.


RV7Ron 01-29-2010 01:19 PM

I'm also at this point and dont have an answer yet how I'm going to do it so I'll be interested to see what the concensus is?

apkp777 01-29-2010 01:36 PM

I am "all ears" on this one too. My plan was to use regular (non-etching) primer on the fiberglass. However, I have a couple cans of self-etching left that I wouldn't mind using up.

Ted Johns 01-29-2010 02:09 PM

If it was me, I'd use an epoxy primer like PPG DP40 or similar. Don't go wild with the amount of coverage, because down the road you will have to scuff it all with red scotchbright and give it another DP40 coat prior to the finish coats.

Most epoxy primers have only a day to a week of "open" time before the above requirement kicks in. When the primer coat is too old, the top coats don't chemically bond to it, and peeling may happen.

rocketbob 01-29-2010 02:43 PM

Kirker Enduro prime. Etching primer, works on both aluminum and fiberglass, and is inexpensive. I've used it and really like it, lays down smooth. Equivalent to PPG DP40LF. You can get two gallons sprayable for the price of two quarts of PPG.

RV7Ron 01-29-2010 02:43 PM


Originally Posted by Ted Johns (Post 401195)
If it was me, I'd use an epoxy primer like PPG DP40 or similar.

Epoxy primer on both alum and fiberglass? Acid etch alum first?

What about filler primer for the tips? Would that go on over the epoxy primer?

Ted Johns 01-29-2010 03:47 PM


Originally Posted by RV7Ron (Post 401208)
Epoxy primer on both alum and fiberglass? Acid etch alum first?

What about filler primer for the tips? Would that go on over the epoxy primer?

I don't like the idea of acid etch sitting in between built up layers, so I'd reserve it for individual parts. Built up parts I'd carefully solvent wipe and then scotchbright etch. If the parts are already alodined, solvent wipe and shoot primer.

I don't like the idea of etch primer on fiberglass, though some mfg's may be ok with it.

Fiberglass: Fill pinholes first (resin wipe or wonderfil, or ...), then epoxy prime, them filler primer as needed. If you're going to give it a full coat of urethane filler primer, and not sand through it, then no need for the epoxy primer. The main advantage to a epoxy primer under a filler primer is that you immediately see when you've sanded through, and the epoxy is a bit tougher to sand through.

PPG's optimum plan for aluminum is a thin coat of acid etch, followed by a coat of epoxy primer. Many of their topcoats are not recommended for direct application over DX1791 (their chromate based etch primer). The DX1791 largely replaces alodining by providing some chromate based corrosion protection. Yup, it turns the metal a nice light gold color. If you don't need the extra bit of corrosion protection, epoxy straight to prepped aluminum is fine.
People have been successful shooting PPG Concept straight over DX1791, but when in doubt, follow the p-sheets, that's what I say.

rv9aviator 01-31-2010 10:48 AM

This is a related question to this thread as I am fitting my elevator tips right now too. I am using West systems epoxy and 410 micro to fair the surfaces. after pop riveting the tip on I taped the seam with .007 2 inch wide tape then mixed the 410 filler and faired the surfaces. I am satisfied with the job except for priming then using a little fast build primer to finish the job. West systems calls for a barrier coat of just mixed resin rolled on and tipped off after the fairing work is done. Can I replace this step with PPG DP-40 epoxy and then go over that with the fast build primer. I'm thinking the rolled on epoxy will be much harder to get back smooth. Thanks

broken1234 02-01-2010 08:45 AM

Ted is spot on with the PPG suggestions... I would only add that we had PPG do some testing for us a few years ago and they determined that a layer of DP48LF reduced as a sealer would penetrate the pinholes and scratches on a composite part a bit better than a filler primer, thus providing a slightly better surface for the thicker products to grip. Be sure to use the DP401LF hardner, by the way.

On an RV-10 fuselage, for instance, we'll carefully apply DX1791 to the metal surfaces, then coat both metal and fiberglass with DP48LF, then continue on with whatever filler primer is necessary to get the fiberglass properly prepared for topcoats.

Our background is mostly Lancair work, and we have found that the typical RV project needs a whole lot more TLC to the fiberglass areas than the metal... ;-) But I suppose that comes from experience, correct?

As for the DP vs. epoxy issue over the West filler, you should be OK with this substitution. As a rule, I would do a test coupon first to confirm this, but I have never had a problem with West/PPG epoxy combinations.

Brad Simmons
Airframes Inc.
Milan, TN

rv9aviator 02-01-2010 09:45 AM

Thanks for the tips Brad and Ted. :)

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