Sealing Faying surfaces and exterior seams
Skip this show me the data sheet for CS 2415 (Edge Sealant)
Skip this show me the data sheet ......PS870B (Lap Joint Sealant).
Material Name: PS870B
Description: 2 part , dichromate curred polysulfide compound. Elcellent adhesion to common aircraft substrates.
Details: It has a service temperature range from -65°F (-54°C) to 250°F (121°C), with intermittent excursions up to 275°F (135°C). This material acts as an effective barrier against the common causes of corrosion on aluminum alloys or between dissimilar metals. The cured sealant maintains elastomeric properties after limited exposure to both jet fuel and aviation gas. It is a two-part, manganese dioxide cured polysulfide compound. The uncured material is a low sag, thixotropic paste suitable for application by extrusion gun or spatula. It cures at room temperature to form a resilient sealant having excellent adhesion to common aircraft substrates.

Do I need to seal the exterior seam of the aircraft with CS2415?
What about the lapjoint with PS870B?
I’m sure you have heard the old adage “pay me now or pay me later” It’s all about corrosion control, let’s take a look at a simple lap joint, which is what we are talking about. here. Click the first graphic to expand and see predicted sheet opening under stress.

The Graphic below will show some factors that may cause the lap joint to open or spread apart as the aircraft experiences stress in flight. Take a close look and you will see under a load the joint is going to move
. This is normal. Notice how EXHAUST, moisture, etc could get in the lap joint if the paint was cracked. I will talk about this very important aspect "paint" later in this article.

Click next graphic to see additonal factors affecting opening.

Fretting Corrosion
A particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris.

Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. [Figure 6-10] The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion (on bearing surfaces) has also been called false brinelling. In A&P school we were taught to always inspect lap joints for bulging of skin surface, which may indicate the presence of corrosion between the faying surfaces.

[ Figure 6-10 ]

PREVENTING Lap Joint corrosion.

One of the best methods of preventing corrosion in fuselage lap joints is to seal the lap joint in some way to prevent vapors and liquids from entering . A sealant (PS870B) is generally applied to the mating surfaces of the lap joint prior to assembly. However, sealants can degrade over time. Regular inspection of airframes, particularly older airframes, is required to identify corrosion when it occurs and replace the corroded material.

Most of us don't bother to put a sealant on the lap joint of our experimental aircraft before we rivet it but it is very easy to apply a sealant to the exterior seam. And we should.

External Joint sealant.
CS-2415 it is designed for this purpose! It has the ability to expand.

Lap Joint Fretting surface Sealant
PRC-Desoto PS-870B-1-2 Class B Corrosion Inhibitive Sealant

Both can be purchased from

Now I said I would get to another important aspect of protecting your lap joint from corrosion and that is PAINT. Look at the graphic showing the elasticy of paint after 8 years. We know the CS 2415 will expand but how long will your topcoat last and protect you as well?

Below is a typical paint scheme.

Paint normally last 6 to 8 years. However if it's kept inside I find this research a little hard to believe, but the point is after paint failure corrosion starts accelerating.

Notice the lap joint. A typical location of paint failure.

Starting to see the need for an elastic sealant!

1.) Seal Fretting surfaces With

PRC-Desoto PS-870 Class A Corrosion Inhibitive Sealant - MIL-PRF-81733D

2.) Seal the edges with CS 2415
3.) Use a good top coat paint
Check out the Knowledge base on corrosion control and priming for more information on those subjects.