Corrosion Control - Sealants and/versus Primers

Lee H. Erb, EAA Chap 1000 Det 5, Arlington TX; EAA Chap 34

Originally published July 1997

Introduction

You will have to make your own design decision on what sealant or primer you use. The information here is to give you an idea what is "out there" so that you can frame your questions to test sales persons.

What is a sealant and what is a primer gets pretty intertwined. You sometimes have to "go with what you can get." I try to make a distinction based on the purpose. If it is primarily to seal, it is a sealant. If the material is used to improved painting, it is a primer.

The grouping of various purposes are listed below. This is a portion of data sheet Table of Contents from Courtauld Aerospace publications. At least at my company, the perception is that if they don't have a sealant or a primer for a given purpose, then no one does. (Each Category may have 25 or more specific formulations for sale.)

Sealants

Fuel Tank, Pressurized Cabin
Corrosion Inhibitive
Cabin Pressurization, Aerodynamic
Windshield
High Temp, Firewall
Fuel Tank Coating
Adhesion Promoters and Primers
Electrically Conductive

Primers

Epoxy Primers

General Purpose, Commercial
Non-Chromate and Lead Free
Military Specification
Urethane Compatible

Polyurethane Primers

Elastomers

Flexibility

Either sealant, primer, or paint on sheet metal structures must have flexibility because the structure "breathes." Especially around fasteners and latches there are local stresses.

Epoxies were a big boon in industry over Zinc Chromate when they became readily available. Then EPA was formed and they were really needed. Polyurethanes are even better and their cost is less of a factor now.

Classes of Sealants

There are three classes of sealants:

Class A - Brush Grade
Class B - Fillet Grade
Class C - Faying Surface Grade

I think they are self-descriptive. Paint primers, I believe, fit in Class C. Not all formulations will have all three Classes.

Problems

In my mind, the biggest problem for the Homebuilder using sealants is the sealants have a relatively short shelf life. One is as short as 2 months from manufacture. Obviously those would not be used. Six months shelf life for a small amount is not too bad. Expense and availability may also be opportunities for solutions.

Selection

I said before, "It your choice." If I were trying to do an excellent control of corrosion at the least cost and simple application, I would choose a "good ole" MIL-S-8802 for faying and filleting aluminum structures of the homebuilt category. In areas of EMI, static electricity, and lightning protection, I would go with a corrosion inhibitive, electrical conductive sealant for faying surfaces. This type can get expensive.

For a primer, I would go with a urethane compatible epoxy. At least with a low VOC epoxy primer. I don't know how this fits in with the Stits Process but I bet someone else can tell you.

Don't do like one of my idols, Steve Whitman, and fall into the "I have always done it this way" trap. I will always remember Steve Whitman for his accomplishments and how he help me dream aviation, but I am sure that he will have saved countless peoples' lives at the expense of his and his wife's by his one "mistake."

Steel and Dissimilar Metals

One of these times we will get into corrosion control and sealing of steel and dissimilar metals. There have been many changes and new approaches since I was actively involved.

(The folks at Poly Fiber have a wide variety of primers available for your use. Stits primers are used exclusively at the Flying Snake Ranch. Poly Fiber has also developed a 4-step Composite Finishing Process that is completely NON-HAZMAT. It sounds really good for you plastic airplane buffs. George Gennuso owes The Leading Edge a report on his experiences with this system. Rumor has it that Poly Fiber will soon be releasing a NON-HAZMAT primer and fabric finishing system, but we won't see it until it has been sufficiently beta tested. Check it all out at http://www.polyfiber.com or call them at 800-362-3490 and ask for the free info pack. Or, shucks, just cruise down to Flabob and visit them at the plant. You might want to call and warn them first. Wear your distinctive Chapter 1000 Project Police Tactical Assault Force T-shirt for extra brownie points. -- ed)

EAA Chapter 1000 Home Page

E-Mail: Web Site Director Russ Erb at erbman@pobox.com

URL: http://www.eaa1000.av.org/technicl/alodine/sealants.htm
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 22 December 1997