Fuel and pressure systems in aircraft demand sealants that survive fuel immersion, vibration, temperature cycles, and pressure spikes without cracking or leaking. This guide presents the Top 10 Aerospace Sealants and Application Techniques for Fuel and Pressure Systems in workshop ready language. We cover chemistry selection, surface preparation, mixing discipline, bead application, cure control, inspection, and rework so joints perform in service. Each numbered section explains why the practice matters and gives steps you can apply on the hangar floor. Follow standardized methods, practice clean habits, and document each step for traceability. The result is safer aircraft, fewer leaks, lower rework, and longer component life.
#1 Sealant Chemistry Selection
Match chemistry to service fluid, temperature range, and structural movement for durable performance. Polysulfides remain the staple for integral fuel tanks because they resist fuel, maintain flexibility, and bond to treated aluminum. Fluorosilicones fit Skydrol and aggressive hydraulic fluids where standard polysulfides may soften. Silicones suit high temperature zones near engines, but fuel compatibility must be proven by data. Two part pastes provide performance and shelf control, while pre mixed and frozen packages improve consistency and reduce mixing error. Qualify selections using applicable specs, immersion data, and hardness windows. Do not mix brands within one joint. Record batch, lot, and expiry.
#2 Surface Preparation Mastery
Clean, abrade, and clean again using a validated sequence tailored to the substrate. Remove oils with approved solvent wipes, using the two cloth method to avoid redepositing contaminants on the workpiece. Light abrasion increases surface energy and removes oxide or conversion residue. Vacuum and lint free wipe after abrading to clear dust from corners and fastener lines. Apply the correct primer within its recoat window, measuring film thickness and honoring full flash time before sealing. Mask adjacent areas to control spread and oversqueeze. Record humidity, temperature, and dew point. The best sealant cannot rescue a poor surface, so preparation receives priority.
#3 Mixing, Ratio, and Induction
Accurate ratio yields full polymerization and the flexibility intended by the design. Use a calibrated scale or a cartridge system with positive displacement pistons for repeatability. Scrape container walls and mix until a streak free color and uniform texture are achieved. Honor induction time when specified, since entrapped volatiles can create bubbles or voids under pressure. Degas in a vacuum chamber for bubble sensitive joints and long beads. Log start time because work life begins at first contact between base and curing agent. Discard material at end of pot life even if it still appears workable.
#4 Bead Geometry and Tooling
Consistency of bead shape controls squeeze out, bondline thickness, and the risk of hidden voids. Select nozzle size to match the designed fillet radius and joint width. Keep the tip at a constant angle and push the bead to wet the surface rather than pulling it. Overlap starts and stops to prevent holidays and pinholes. Tool the fillet with a wetted spatula to expel trapped air and to form a smooth aerodynamic profile. Maintain continuous pressure in pneumatic guns for even delivery. Replace worn pistons and seals to avoid pulsation and erratic flow.
#5 Cure Control and Environmental Conditions
Cure profiles for two part polysulfides and silicones depend strongly on temperature and humidity. Post cure steps may be required before pressurization or fuel wetting to avoid early swelling. Use calibrated thermometers and hygrometers at the joint location, not at a distant wall. Record partial cure checks such as Shore hardness and solvent rub to verify progression. Protect wet sealant from dust with breathable covers and controlled access. Observe full cure time before leak testing or fueling. If the environment drifts outside limits, pause work and tag the area until conditions stabilize.
#6 Bondline Design and Sealant Access
Good engineering design reduces stress concentration and makes application feasible in tight bays. Provide minimum bond widths, generous fillet radii, and access holes that allow the correct nozzle angle. Avoid sharp corners and knife edges that crack during vibration or thermal cycling. Use compatible spacers to control thickness in structural joints and to prevent squeeze out starvation. For blind areas, employ inspection mirrors, borescopes, or guided end effectors. When access is impossible, consider pre cured gaskets or form in place seals with intelligent fixturing. Document deviations and obtain approval before altering geometry or sequence.
#7 Integral Fuel Tank Sealing Practices
Plan the sequence so that fastener wet installation, faying surface coverage, and external fillets occur in one controlled flow. Butter fasteners and holes to exclude capillary paths around shanks. Spread sealant on overlaps before bringing parts together to avoid dry spots. Maintain clean glove discipline, change gloves often, and protect tools from fuel residue. Use daylight and mirror checks to verify coverage behind ribs and stringers in shadowed locations. After cure, perform pressure decay or vacuum box testing using calibrated gauges. Archive results with photographs and location maps for future maintenance reference.
#8 Pressure System Joints and Fittings
Pressure lines and manifolds require zero leak performance under cyclic mechanical and thermal load. Use thread sealants or anaerobic compounds only where specified, since overuse can block small orifices. For flared or flanged connections, apply a thin uniform layer on the mating surfaces and avoid bead squeeze into the bore. Align tubing to remove side load before tightening hardware. Torque to specification with calibrated wrenches and record values in the work order. Mark fittings with witness paint for movement detection. Reinspect after thermal cycles during ground run and again after the first flight.
#9 Inspection, Non Destructive Evaluation, and Acceptance
Inspection starts before application and continues through cure and test. Verify primers, mix logs, bead geometry, and environmental readings at the point of work. Use borescopes to spot holidays, voids, and poor wetting in hidden corners. Employ dye penetrant only when compatible with the substrate, primer, and sealant system. Acceptance criteria should cover fillet radius, adhesion, porosity limits, and visual surface defects. Record nonconformances with clear photographs, dimensions, and measured data for traceability. Release the system to leak testing only when all checks pass and documents are complete.
#10 Rework, Removal, and Health Safety
Plan for safe removal since rework is part of lifecycle maintenance on tanks and lines. Use plastic scrapers and approved chemical softeners to protect substrates and coatings. Mask fuel bay openings and remove debris with a HEPA vacuum to prevent foreign object damage. Re prep the surface fully before reseal by repeating the validated preparation sequence. Maintain ventilation and use the correct respirators and gloves according to the safety data sheet. Train personnel on mixing hazards, skin exposure, and waste handling requirements. Keep traceable records of removed material and new batches to maintain configuration control.