Aerospace Sealants: Specs, Dash Numbers and Buying
Aerospace sealants are two-part or one-part rubber compounds bought to a specification, not a brand: AMS-S-8802 and AMS 3281 for polysulfide fuel tank sealant, AMS 3277 for polythioether, MIL-PRF-81733 for corrosion inhibiting sealant and AMS 3374 for silicone firewall sealant. The class and dash number set how it is applied and how long you have to work it.
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Sealant types and where they are used
Pick the chemistry from the job, then the class from how it goes on. Fuel tanks and pressure structure use fuel resistant polysulfide or polythioether. Faying surfaces and fastener installations on corrosion prone joints use a corrosion inhibiting compound. Firewalls use silicone. General purpose bonding and sealing of electronics or ducting often uses a one-part silicone RTV.
| Family | Chemistry | Typical use | Governing spec |
|---|---|---|---|
| Fuel tank sealant | Two-part polysulfide, room temperature cure | Integral fuel tanks, fillet and brush coat sealing, fuselage | AMS-S-8802 |
| Low density fuel tank sealant | Two-part polysulfide, low specific gravity | Integral fuel tank and fuel cell cavities where weight matters | AMS 3281 |
| Fast curing fuel tank sealant | Two-part polythioether | Integral fuel tanks and general purpose, intermittent use to 360 °F | AMS 3277 |
| Corrosion inhibiting sealant | Polysulfide (Class 1) or polythioether (Class 2) with chromate or non-chromate inhibitors | Faying surfaces, wet installed fasteners, coating of metal components | MIL-PRF-81733 |
| Firewall sealant | One-part or two-part silicone | Firewall structures against air, vapor and flame | AMS 3374 |
| General purpose RTV | One-part silicone, moisture cure | Adhesive sealing where fuel resistance is not required | MIL-A-46106 |
Fuel tank sealant is also the material that ends up around fluid fittings and seals, so check compatibility with the O-rings and the fluid in the system before substituting a chemistry.
The specifications that govern aerospace sealants
Most sealant callouts point to one of six documents. Status below is what the issuing body or a standards reseller showed when we checked; confirm the revision your drawing invokes before ordering.
| Spec | What it covers | Status as checked |
|---|---|---|
| AMS-S-8802 | Fuel resistant, two-component polysulfide sealing compound that cures at room temperature. Type I dichromate cured, Type II manganese dioxide cured. | Revision F current per a distributor technical page. MIL-S-8802 was superseded by SAE AMS-S-8802 in May 1999. |
| AMS 3277 | Polythioether rubber, fast curing, for integral fuel tanks and general purpose, intermittent use to 360 °F (182 °C). Two types and three classes. | Revision L, 28 June 2024, listed as current by BSI. |
| AMS 3281 | Polysulfide (T) synthetic rubber for integral fuel tank and fuel cell cavities, low density, intermittent use to 360 °F. Three types and three classes. | Revision G, 13 June 2022, listed as current by BSI. |
| MIL-PRF-81733 | Sealing and coating compound, corrosion inhibitive. Four types by application method, two classes by base polymer, two grades by inhibitor. | Revision D (15 May 1998) shown as active. It superseded MIL-S-81733C. We found no document numbered AMS-S-81733. |
| AMS 3374 | Silicone sealing compound for aircraft firewalls, six types, one-part or two-part, room temperature cure. | Revision G, 22 May 2023. |
| MIL-A-46106 | One-part RTV silicone adhesive-sealants in three groups, moisture cured, plus primers. | Revision B (1989) with Amendment 3 (1992) shown as active. |
One more document sits behind the packaging and dating of these materials: SAE AS5502, Standard Requirements for Aerospace Sealants and Adhesion Promoters, which MIL-STD-129 cites when it sets shelf life marking for sealant kits. See shelf life control for how that marking works.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Class and dash numbers: application time decoded
The letter and dash after a sealant spec tell the applicator how the material goes on and how many hours it stays workable. Ordering the wrong dash number is the most common sealant mistake, because a B-1/2 and a B-2 are the same chemistry but behave very differently on the floor.
AMS-S-8802
- Class A is brushable, with application times A-1/2, A-1 and A-2.
- Class B is for extrusion gun or spatula, with B-1/2, B-1, B-2 and B-4.
- Class C is for faying surface sealing. Its designations carry two numbers, for example C-8(20), where the number in parentheses is the assembly time in hours.
For Classes A and B the dash number is the minimum application time in hours. A distributor technical page gives the example of a Type I, Class A-2 sealant having an application time of 2 hours.
MIL-PRF-81733D
The specification states the designation scheme directly. Type I is for brush or dip, Type II for extrusion by gun or spatula, Type III for spray gun and Type IV for faying surface application by gun or spatula. Class 1 is polysulfide based and Class 2 is polythioether based. Grade A contains chromate corrosion inhibitors and Grade B contains non-chromate inhibitors. The dash number is the minimum application time in hours: Type II Class 1 is offered as -1/6, -1/4, -1/2, -2 and -4, and Type IV Class 1 as -12, -24, -40 and -48. The spec's own example reads Type I-1/2 as a brushable material with half an hour of application time.
A real designation, read left to right
Distributor listings describe PPG PR-1440 B-1/2 as qualified to AMS-S-8802 Type II, Class B-1/2. Read it as: Type II, manganese dioxide cured polysulfide; Class B, applied by extrusion gun or spatula; dash 1/2, a minimum application time of half an hour. The same listings give the application life as 30 minutes. AMS 3277 uses a similar pattern, and a Boeing Distribution listing shows the form AMS 3277 Type II, Class B-1/2, Grade 1. The SAE revision history notes that AMS 3277K added Class C-12(48) and C-48(168).
Mixing, kits and cure
Two-part sealants are sold as matched kits, and the kit is part of the specification. Do not let anyone split a kit or mix by eye.
MIL-PRF-81733D sets the unit of issue. Can kits come in 1/2 pint, 1 pint, 1 quart and 1 gallon sizes holding 6, 12, 24 and 96 fluid ounces of base, with the curing agent packed in a separate jar at the manufacturer's mix ratio and the headspace purged with nitrogen or carbon dioxide. Sectional cartridge kits for Types I, II and IV come in 2-1/2 ounce and 6 ounce sizes conforming to MIL-S-38714. Every component container must be marked with the date of manufacture and the date of packaging.
Three times matter on the floor:
- Application time, set by the dash number: how long the mixed material stays usable.
- Tack-free time, tested and limited by table in the specification: when the surface stops picking up dirt and can be handled.
- Cure time: when the material reaches its specified hardness and properties. MIL-PRF-81733D defines a standard cure for its Class 1 Types I and III of 7 days at standard conditions. A distributor listing for PR-1440 gives full cure as 14 days at 77 °F.
Mix ratio is by weight and is fixed by the manufacturer; one PR-1440 listing gives 10:100, Part A to Part B. Large users often buy sealant premixed and frozen in cartridges so the mechanic thaws and applies without mixing. MIL-STD-129 recognizes this practice for select Federal Supply Group 80 items, noting that industry batch produces and freezes to prevent degradation and that the shelf life of those items starts at the date of shipment. Frozen sealant has to stay frozen through shipping and receiving, which makes cold chain records part of the paperwork.
Materials and trade-offs: chromated, non-chromated, weight and temperature
Chromium shows up in two places in sealant specs. In AMS-S-8802, Type I is dichromate cured and Type II is manganese dioxide cured, so the type choice decides whether the cure system carries chromium. In MIL-PRF-81733D, Grade A uses chromate corrosion inhibitors, while Grade B uses non-chromate inhibitors and must contain no chromium or lead compounds. Programs moving away from hexavalent chromium usually change both, but the change is an engineering decision: corrosion qualification data on legacy structure may assume the chromated grade.
| Trade-off | What the specs say |
|---|---|
| Temperature, polysulfide | AMS-S-8802 continuous service is given as -65 to +250 °F by a distributor summary; AMS 3281 allows intermittent use to 360 °F. |
| Temperature, polythioether | AMS 3277 allows intermittent use to 360 °F. MIL-PRF-81733D Class 2 (polythioether) covers -80 to +320 °F continuous, against -65 to +250 °F for Class 1 polysulfide. |
| Weight | MIL-PRF-81733D caps cured specific gravity at 1.65 for Class 1 and 1.50 for Class 2. AMS 3281 exists specifically for low specific gravity polysulfide. |
| Firewall | AMS 3374 sealants are rated from -65 to +400 °F and to withstand flash temperatures up to 2000 °F. |
Sealant also interacts with finishes. Corrosion inhibiting sealant is usually wet installed over primed or conversion coated surfaces; the finishing side is covered on surface finishing.
Firewall and silicone sealants
Silicone does not belong in a fuel tank, but it is the right answer at a firewall and in many general sealing jobs. AMS 3374 covers six types of silicone firewall sealant. Type 1 is a one-part, or pre-mixed and room temperature stable two-part, high temperature silicone cured by condensation. Type 2 is a two-part high temperature silicone cured by addition. Name the type on the PO, because the cure chemistry changes how the material behaves in thick sections and enclosed joints.
MIL-A-46106 covers one-part RTV silicone adhesive-sealants that cure by reacting with moisture in the air, in three groups, and it also covers the primers used with them. Moisture cure means thick beads cure slowly from the outside in, so check the manufacturer's depth of cure data before using a one-part RTV to fill a deep cavity.
Ordering checklist for sealants
Put the full designation on the PO, not a product name alone, and say what paperwork must come with it.
- Specification and revision, type, class, dash number and grade, for example MIL-PRF-81733D, Type II, Class 1, Grade B, dash 1/2. MIL-PRF-81733D itself tells acquirers to state type, class, grade and application time.
- Qualified source: MIL-PRF-81733 awards go only to products qualified for QPL-81733. Ask for the QPL listed product name and manufacturer.
- Kit form and size: can kit, sectional cartridge, premixed frozen cartridge, and quantity per unit.
- Color if other than as manufactured.
- Manufacturer batch number and date of manufacture on every unit, plus the expiration date.
- Minimum remaining shelf life at receipt, stated in months.
- Certificate of conformance tracing to the manufacturer and batch, and a manufacturer's test report when your program requires one. See certificates of conformance.
- Shipping condition: frozen with dry ice and temperature record for premixed material, or ambient within the manufacturer's storage limits.
- Any prime material specification that overrides the industry spec.
Counterfeit and quality risks, and how to receive sealant
The risk with sealant is rarely a fake chemistry. It is old material, broken cold chain, relabeled dates and substituted dash numbers. A kit that has gone past its date may still mix and look normal, then fail tack-free or hardness requirements in the tank.
- Dates. Check date of manufacture, batch and expiration against the certificate. Material that was repacked or decanted into new containers has lost its manufacturer traceability.
- Dash number. Confirm the class and dash on the label match the PO. A B-2 shipped against a B-1/2 order changes cure schedules for the whole job.
- Qualification. Confirm the product is listed for the spec, type and class you ordered.
- Frozen material. Look for dry ice remaining and a temperature record. Material that arrives thawed should be quarantined until the manufacturer or your engineering approves it.
- Kit integrity. Base and curing agent must be from the same kit and unopened.
Our receiving checklist is on receiving inspection, and the wider controls are on counterfeit parts avoidance.
How we source sealants
We do not stock sealant or sell it from a catalog. We source the qualified product through authorized distributors or the manufacturer, flow your spec, revision, dash number and shelf life requirement down on the order, and check the batch, dates and certificate before anything ships to you. Where a job needs frozen premixed cartridges, we specify the cold chain on the order. We can put sealant on the same PO as the machined parts, fasteners and seals it goes with, or build it into a kit. When a legacy callout names a discontinued product, we look for a currently qualified product to the same spec and type and bring the substitution to your engineering for approval rather than deciding it ourselves. Older or hard to find items go through our obsolete parts process.
Related pages: structural adhesives, greases and lubricants, and the chemicals category.
Get sealant sourced with the right dash number and paperwork
Send the full sealant callout, quantity, kit form and the minimum shelf life you need at receipt. A person replies within one business day with a source and the document package you will receive, and we can combine the sealant with the hardware it goes into on one order. Request a quote for sealants and hardware. Back to all parts.
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Questions
What does the dash number on a sealant mean?
For AMS-S-8802 Classes A and B and for MIL-PRF-81733, the dash number is the minimum application time in hours, meaning how long the mixed sealant stays workable. A B-1/2 gives about half an hour, a B-2 about two hours. AMS-S-8802 Class C designations such as C-8(20) carry a second number in parentheses, which is the assembly time in hours for faying surface work.
What is the difference between AMS-S-8802 Type I and Type II?
Both are two-part polysulfide fuel tank sealants that cure at room temperature. Type I is dichromate cured and Type II is manganese dioxide cured. The type changes whether the cure system contains chromium, which matters for programs reducing hexavalent chromium. Check what your drawing or maintenance manual allows before switching types, since qualification and repair data may be tied to one of them.
Is AMS-S-81733 the same as MIL-PRF-81733?
We found no document numbered AMS-S-81733. The corrosion inhibiting sealing and coating compound specification we could verify is MIL-PRF-81733, revision D dated 15 May 1998, which superseded MIL-S-81733C and is shown as active. If your drawing cites a different number, send it to us and we will confirm what document it refers to before quoting.
Should I choose polysulfide or polythioether sealant?
Follow the drawing or manual first. Where you have a choice, polythioether specs such as AMS 3277 are written for fast curing and intermittent use to 360 °F, and MIL-PRF-81733 Class 2 polythioether covers -80 to +320 °F continuous against -65 to +250 °F for Class 1 polysulfide. Polysulfide has the longer qualification history on legacy structure. Weight, cure schedule and existing repair data usually decide it.
Can sealant be shipped premixed and frozen?
Yes. Premixed frozen cartridges are common for production lines, and MIL-STD-129 notes that for select Federal Supply Group 80 items industry batch produces and freezes the material, with shelf life starting at the date of shipment. The cold chain then becomes a quality requirement: specify frozen shipment on the PO and quarantine any material that arrives thawed until it is approved.
What paperwork should come with aerospace sealant?
At minimum a certificate of conformance naming the specification, revision, type, class and dash number, the manufacturer, the batch number, the date of manufacture and the expiration date. For qualified products, the product should appear on the qualified products list for that spec. Many programs also require the manufacturer's batch test report. State all of this on the PO so it is flowed down.
Related
Sources
- MIL-PRF-81733D, Sealing and Coating Compound, Corrosion Inhibitive (EverySpec)
- Silmid: SAE AMS-S-8802 specification summary
- BSI: SAE AMS 3277L polythioether sealing compound
- SAE Mobilus: AMS 3277K revision record
- BSI: SAE AMS 3281G low density polysulfide sealing compound
- Standards Norway: SAE AMS 3374G firewall silicone sealing compound
- MIL-A-46106B, Adhesive-Sealants, Silicone, RTV, One-Component (EverySpec)
- SkyGeek: PR-1440 B-1/2, AMS-S-8802 Type 2 Class B-1/2
- Boeing Distribution: AMS 3277 Type II Class B-1/2 Grade 1 listing
- Boeing Distribution: PR-1440 Class B pint kit listing
- MIL-STD-129R Change 2, Military Marking for Shipment and Storage (EverySpec)
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