Aircraft Priming 101
Priming 101 Vans Aircraft by
Aluminum Priming & Primers Interior
Priming FROM… TEXAS
RV10.com
Updated
12/31/2010
1.) Clean parts (PPG
DX 330)
2.) Etch parts (PPG DX 533) or Alumiprep, same thing.
3.) Corrosion Protect (PPG DX503) or Alodine 1201, same thing.
4.) Prime parts SHERWIN WILLIAMS
CM0724400 Epoxy Primer & CMO724114 Adduct.
For INFORMATION on All Sherwin Williams Paints follow this link… Sherwin
Williams Official Web Site! Best web sites Award! 2010
Sounds simple, but WAIT, not so fast. What if I said primer
needs to be primed? What? Why would you prime a primer? Well
let’s find out…
Some
primers are called Self etching primers which contain a small
amount of acid right in the primer, for example DuPont™ Variprime®. These primers should be applied as a good wet coat as opposed to
several thin layers to allow the acid to etch the metal then the acid flashes
off leaving a good adhering base coat primer. However if using VariPrime, DuPont’s tech sheet calls for sealing (using their sealer)
or priming over their primer with an epoxy primer within 16 hours. VariPrime does not seal against moisture according to DuPont’s tech sheet.
Note: You
can use a self etching primer to skip the etching and
the Alodine process, and there are places you may
want to do this, such as the inside of hard to reach skins.
Don’t Alodine
then use a self etching primer, you will be acid
etching you alodine.
Thus
spraying a self etching primer over an Alodined surface will degrade the quality of the Alodine process. Reading DuPont's recommendations regarding
VariPrime, tells us that VariPrime
is NOT recommended as a finish surface for aluminum. DuPont recommends applying
another primer or finish coat over VariPrime. VariPrime does not create a vapor barrier (it's porous and
doesn't stop water from moving through it). So spray over it with an Epoxy
primer such as Sherwin
Williams CM0724400 or spay your topcoat!
NOTE: There are TWO VariPrimes,
DuPont™ Variprime® 615S™/625S™
Self-Etching Primers. 615S contains corrosion inhibiting zinc chromate, 625s is Lead and
Zinc chromate Free. Use 615s on Aircraft for the
corrosion protection. VariPrime works great for shooting skins, easier then acid
etching and aloding. You don’t need an expensive gun
to prime, a Sharpe FX3000 gun $148.00 in
2010, shooting at 20-30 psi at the gun for a tip psi of 8 to 9 and a tip or
nozzle size of 1.3 or 1.5. Either size works great. Normally primers are shot using a 1.8 tip
size on the spray gun but not the case with VariPrime
or the Sherman Williams CM0724400, they are the easiest paints I’ve ever
sprayed and a 1.3 or 1.5 nozzle works great.
Strontium Chromate - the approved
replacement for Zinc, is found in SHERWIN WILLIAMS CM0724400 and conforms to the MIL Spec23377F.
A good reason to use a MIL Spec 23377 type epoxy primer if you live in a high corrosion
area, is the spec calls for a minimum of 52% of its
pigment to be Strontium Chromate and aluminum corrosion
resistance is required to be better than 1000 hours in a 5% solution salt spray
test. Water resistance is required to be 4 days immersion in 120 degree water
as well as 24 hours in 250 degree lubricating oil!! These primers get much
harder than zinc chromate, and are also resistant to MEK solutions. Their color is a yellow color (the spec calls the raw pigment
"deep yellow"). And the spec calls for the shelf live to be no longer
then 1 year. It might be fine after a year but if you
have problems spraying it, Trash it!
Now with all this said is it really
necessary? Well
that’s where it’s up to you to decide, after all YOU
are the builder!
One employee of Vans stated he used Dupont VariPrime on a boat and 15
years later it’s showing no signs of corrosion, so what about not protecting
from moisture as stated on the data sheet. I don’t know! We can all get carried away with over building
taking months beyond what it should take to finish our project. There are
planes out there flying around with very little corrosion protection if any at
all. With that said, the process I’ll recommend, is to clean the metal and
etch, (Alumiprep or similar), then Alodine, then paint with a two part epoxy such as SHERWIN WILLIAMS CM0724400 using Variprime 625s for inside the skins that might be hard to
acid etch and alodine then topcoat with SHERWIN WILLIAMS CM0724400 Epoxy
Primer!
I use Dupont’s VariPrime for the inside of the skins where it would be
hard to acid etch and alodine. Just spray with a Self Etching Primer and follow up with a two part Epoxy
SHERWIN WILLIAMS CM0724400 or similar.
Remember, prime over your self-etching primer with an Epoxy Primer or
topcoat. The difference in weight from
using just a self etching primer not coated with 2
part epoxy will save you only about 5 pounds (you figure the solids in the
paint) and it's not sealed! Dupont recommends you
seal it with another primer or topcoat.
The real benefit using a self etching primer is on the skins to keep from having to acid etch and alodine!
Last
updated 8-25-2010
David J Murphy is an A&P and Commercial Pilot for a major airline.
To read more on corrosion control follow
the link below
to EAA's corrosion control index
article.
http://www.eaa1000.av.org/technicl/c...on/corridx.htm