Primer war...not really...just a question
 

What primer is best to use on the interface between aluminum and fiberglass. As I have done the empenage fairings I have elected to fill/fiberglass/fill the seam. There is exposed sanded aluminum and I am years away from final paint and want to protect the bare aluminum

Thanks

Mark, consider me a very interested bystander.

I'm at same point, with same concerns. I was going to use a polyester sandable primer that was recommended by other -10 builders and available in quart orders. Turns out it's probably very risky getting any onto the edges of transparencies, so I've postponed its use for now and will probably roller it on if I do use it, so I have total control of where it ends up. Still raises some concern over how to handle the inevitable overlap of sandable primer on FG and etching primer on aluminum surfaces, which is even more of a question in my mind if one is a polyester and one an epoxy product.

I'll wait to see what responses you get, but I have a feeling I will shoot an epoxy high-build primer on the FG transitions and a traditional primer (whatever that is) over the metal / under the color coat.

Talk with whomever is going to paint your RV-10. They will have a strong opinion on the subject. I?ve known some painters that will remove all builder applied primer before painting the aircraft. Why go through the expense and labor if the painter is going to rework everything so they can warranty their work.

I too will fly before final paint (simply missed the painting window this year).

What I have done on other builds:
- Aluminum inside surface is primed, just like the rest of the airplane.
- Fairing installed per plans.
- Fairing to skin gap filled with micro and sanded flush. Same on the pop rivets.
- I single bid of glass over the fiberglass to skin junction, extending over the pop rivets.
- A skim coat of micro on top of that, then final sand.
- Mask off and prime the fiberglass and perhaps 3/4? of the skin (as in prime the area just worked).
- I use the same primer for everything for three planes so far, PPD DP-40LF. As I use PPG products for final paint it all works for me. I never found a need for high build primers but if you want PPG has that as well (PPG K38).

As you will be flying for sometime before paint, go ahead as shoot a single coat of PPG single stage paint on top of all the fiberglass (I use a medium blue - goes well with the bare aluminum). It only takes a few minutes after you prime. It will look like a finished airplane and when ready for final paint some wet sanding and you are ready to go.

Carl

Thanks...that probably the best advice

Interesting. Can you shoot DP-40 outside with just a filter mask like in the old days, or is it cyanide based etc?

....thinking about exterior quick primer vs bare AL to get me flying but not full up paint till much later.....

yes
 

That primer is epoxy based and doesn't use cyanoacrylates, so a regular respirator should be fine...according to the paints data sheet...

In my experience, Epoxy primer has the strongest grip and therefore the preferred first layer. Many support direct application on Aluminum, but check first. I use SPI and it is a direct to metal primer that grabs very good on aluminum. It also sticks to F/G very well and is a go to primer for the Vette guys.

The key with adhesion to aluminum is application within a couple of hours of abrading. Aluminum begins to oxidize the minute you take off the old layer of oxidation. Most don't realize this, as it is not visible like rust. Once the oxidation layer is created, the oxidation usually stops or slows dramatically, as this layer prevents further oxidation. The layer of oxidation is typically VERY thin and not easily recognized for what it is. When you sand AL and watch it turn from dull to shinny, that is oxidized layer being removed. That invisible oxidation layer will allow ANY primer (excluding self-etching primers) to more easily separate, as it is adhered to the oxidation layer, which does not have that strong of a bond with the base aluminum, so be sure it is not there.

Most self-etching primers are acrylic and don't bond nearly as agressively as epoxy primers and therefore worth the effort to use epoxy. Also. self-etch only breaks down the oxidized layer. It does not create the strength provided by mechanical adhesion (requires mechanical abrasion) and therefore sanding or scuffing is still required for a good bond with self-etch. The Previous owner of my kit just sprayed it on, not understanding that, and some of it will literally blow off with compressed air in spots.

Some types of etching, such as alodine, actually do create a chemical bond as strong as mechanical adhesion, but I do not believe you will find that in any self-etch paint. Those chemicals typically require a rinse and therefore wont be found pre-mixed with paint.

Bill, many people apply catalyzed polyester products directly to metal, following the abrasion guidelines I mentioned above, with good success, assuming it is top coated with something that seals it from moisture (the poly products will absorb and transfer moisture to the metal, allowing it to oxidize underneath it, breaking down the bond). However, the best practice is to start with a layer of epoxy primer and apply poly products on top of it for the best adhesion (you get some cross-linking if applied within the epoxies window).

Larry

Cyanoacrylates are a type of anaerobic adhesive (e.g. supeglue). I think you meant Iso-cyanates. They are the nasty element in two part urethane paint activators that you must protect yourself from.

Larry

Glad to hear SPI is getting some use. I've used it on cars and professionally on industrial equipment and it works great and is extremely easy to spray. I used it in an engine bay and it's been highly scratch resistant and hasn't been affected by gas, oil, or brake fluid.

I like to use it before filler and after as a sealant & light pore filler. It can also be used as a seal coat when thinned a bit.