Why primer, and not just paint???

cjensen · Post by **cjensen** » Sat Mar 24, 2007 9:47 pm

Well, the subject pretty much says it all, but what's the deal here?

I decided to just paint my seat ribs with regular old Rustoleum Satin White today, and I'm going to continue using regular paint in the cockpit.

My question is, why don't we just use paint instead of primer everywhere? Seems like it would produce a more protective layer than the self-etching primers. I don't know about how the epoxy primers would compare since I don't use them.

What piece am I missing here?

captain_john · Post by **captain_john** » Sat Mar 24, 2007 9:57 pm

Chad,

Primer does several things that top coats do not. Likewise, the other way around.

I suppose a better question is, "How is primer DIFFERENT than top coats - and sealers and clears too, for that matter... but one step at a time.

What is a good primer? Well, we want something that will stick well. It is our foundation layer. If it fails, so does all that is applied over it.

A self etching primer regardless of manufacturer is usually an inexpensive, dull finish which when cured beyond tack but not for more than 24 hours will accept a top coat readily without additional prep.

Skipping a step here (sealing) and going right to a top coat now... A top coat is designed for a smooth , durable finish that will repel most anything (grease, dirt, bug juice) and look good doing it. They are not designed for application directly to metal, even though it may stick well for an eternity.

That is a a brief overview. Is it what you were looking for?

Questions?

CJ

cjensen · Post by **cjensen** » Sat Mar 24, 2007 10:18 pm

Well, that was a good brief overview, but your last statement brings up a question...

They are not designed for application directly to metal, even though it may stick well for an eternity.

Ok, so if it might stick well for an eternity, and it's durable enough to repel anything (and look good doing it!

), then why do we NEED primer, sealer, before the top coat?

The white on my seat ribs is already nice and durable, and if it essentially "seals", then WHY?

svanarts · Post by **svanarts** » Sat Mar 24, 2007 10:51 pm

It's probably fine. I'd just scuff up the metal a little and spray it on. You can always touch it up in 5 or 10 years if needed. I use very little primer. If it's alclad and it isn't scratched up, it doesn't get primer.

captain_john · Post by **captain_john** » Sun Mar 25, 2007 7:50 am

You missed a word.

It MAY stick well for an eternity.

I didn't say it WILL stick well for an eternity.

Primers are designed to stick to bare metal. Not top coats.

I saw your crosspost, but there are far too many experts over there for me to comment.

Paint "SYSTEMS" have been designed with purpose in mind. You start off with the substrate that is to be coated. What is is made of and what will it be asked to do? Do you want thin light coats (on an airplane, YES!) or heavy marine grade stuff.

Chemical engineers design components of the "system" and then tinker with them to accomodate other environments and substrates (materials).

Let us use our aluminum structures for example. We want thin, light and durable coats for the purposes of corrosion control. Adhesion must be good, but abrasion is not a concern.

What is corrosion? We need to examine the corrosion battery and understand IT before that discussion can continue. Then we can combat it before it starts.

Have you seen the the videos on the ACF 50 website? They explain it really well. Check them out here:

http://www.corrosion-control.com/acf50.html

Soooooo, paint will fail of corrosion gets UNDER IT. If properly applied, the coating will seal out moisture and eliminate a critical component in the corrosion battery.

It is the role of the primer to let the subsequent applications stick and it does it well.

Cheap rattle can stuff has not been tested to any great extent and is very random in it's performance and durability. The stuff is just very random and indeterminite.

It is kinda like getting a puppy. If you get a mutt from the pound, it COULD be a really nice dog and do exactly what you want. It may hunt well for you and be an excellent companion animal with the kids. OTOH, it could be lazy and bite your children when they go to play with it. If you buy a golden from a KNOWN BREEDER who has a long line of quality pups, you will know the adult dog that this little pup will grow into!

CJ

cjensen · Post by **cjensen** » Sun Mar 25, 2007 8:03 am

That's better.

I'm sorta with Scott though. I haven't primed much. Even thought they are alclad, I have primed bulkheads and ribs and all non-alclad pieces. No skins are primed. No primer on the interior, just painted to my color choice.

Thanks CJ.

captain_john · Post by **captain_john** » Sun Mar 25, 2007 8:06 am

Did ya check out the videos? Mebbe Image Air could become a dealer!?!

CJ

cjensen · Post by **cjensen** » Sun Mar 25, 2007 8:10 am

Yeah, I could pass it along...I doubt they would go for it. We don't do a whole lot of paint related stuff, and most of our customers wouldn't do it themselves (lots of "bigger" airplanes...).

captain_john · Post by **captain_john** » Sun Mar 25, 2007 8:16 am

Did it help you to understand the "corrosion battery" and why it is called that and what role the paint plays?

CJ

smittysrv · Post by **smittysrv** » Sun Mar 25, 2007 9:55 am

So if I decide to prime the interior before painting it, do I need to scuff up the interior surface prior to putting on the primer? I haven't ordered my fuselage yet, jes' curious.

captain_john · Post by **captain_john** » Sun Mar 25, 2007 11:49 am

Smitty, yes... you should!

Coating manufacturers agree that a properly prepped surface shall include cleaning (no oils or schmeg) PROFILING (putting little scuffs in it by sanding) the surface and then cleaning again!

The reason for cleaning twice is so you don't force the oils and what not into the grain of the material.

Kinda minuscule, but ya know...

CJ

svanarts · Post by **svanarts** » Mon Mar 26, 2007 9:45 pm

svanarts wrote:It's probably fine. I'd just scuff up the metal a little and spray it on. You can always touch it up in 5 or 10 years if needed. I use very little primer. If it's alclad and it isn't scratched up, it doesn't get primer.

I should qualify what I said there with... "if the paint is designed for primer-LESS application." Some are and some aren't.

Talk about priming metal ranks right in there with religion and politics as a discussion topic.

No one ever agrees on it.

jim_geo · Post by **jim_geo** » Tue Mar 27, 2007 1:16 pm

Scotts got it just right to my way of thinking. Primer does a couple of things. It holds to the sub straight and allows the finish to adhere to the primer. Beyond that though many primer paint combinations are designed to be used together and when used in combination allow the finish material to flow properly and also lay out properly on the project. What I have found is that most finishes will hold a WELL prepped surface fairly well without primer which in most cases is probably what we as aircraft builders would like since we concern ourselves with added weight. For a really great paint job though personally I feel primer is a must. It then becomes one of the compromises we make, as light an aircraft we can make or a really great show stopper that may be some bit heavy.