Marine Sealants Explained: 5200, 4200, 4000UV, Polysulfide, Silicone and Butyl Tape

Ask: will someone need to remove this for service, replacement or rebedding in the next 10 to 20 years? Cleats, stanchions, hatches, transducers, inspection ports, lights and most deck hardware: yes. Choose a removable product (4200-type, 4000UV-type, polysulfide, butyl tape or silicone, depending on the job). Save 5200-type adhesives for joints that are truly permanent.

Below the waterline you need a sealant rated for constant immersion. The 3M 5200, 4200 and 4000UV product pages, BoatLIFE's Life-Calk and Sikaflex-291 all list below-waterline use. Butyl tape doesn't belong below the waterline: BoatUS notes its grip is too modest. Most ordinary silicones aren't rated for it either; a few marine silicones are, so check the label.

• Plastics: this is where most mistakes happen. BoatLIFE says polysulfide shouldn't go on plastics (and especially not polycarbonate), and BoatUS warns polysulfide attacks ABS and Lexan. Sika says Sikaflex-291 must not be used on plastics prone to stress cracking, like acrylic (Plexiglas) and polycarbonate. Silicone and butyl tape are the safest general choices for plastic parts. 3M lists plastics on its 5200, 4200 and 4000UV pages, but test first if you aren't sure what plastic you have.
• Acrylic and polycarbonate windows: use the sealant the window maker specifies. Many common caulks craze or crack these plastics.
• Metal and wood: polysulfide, polyurethane, polyether and butyl all work.
• Near fuel: polysulfide resists fuel best. Polyurethanes and butyl tape do not like fuel or solvents.

Quick picks (typical; always read the label):

• Bolted cleat, stanchion or rail base on deck: butyl tape, polysulfide, 4200-type or 4000UV-type.
• Through-hull or transducer below the waterline: polysulfide or a 4200-type sealant if you may ever replace it; 5200-type only if it's meant to be permanent.
• Fuel fill or vent fitting: polysulfide.
• Light fixture, plastic vent or part you open often: marine silicone.
• Topside seam you want to stay white: a UV-stable polyether or SMP like 4000UV.
• Hull-to-deck joint or rub rail on a rebuild: high-strength polyurethane, per the builder's practice.

New sealant won't bond to old sealant, dirt or wax. Scrape with a plastic blade so you don't gouge gelcoat. For stubborn polyurethane, Boating Magazine's test found a dedicated debonder worked best: score the bead, soak it, wait and pry, repeating. BoatUS warns that old silicone leaves an almost permanent film that blocks new sealant, and it's a common hidden cause of leaks coming back. Scrub silicone residue off thoroughly with the cleaner the new sealant's maker recommends.

If a fitting won't lift after the fasteners are out, Don Casey's BoatUS advice is to assume it's bedded in polyurethane and get a debonder rather than prying, which can tear the deck. If you use heat, keep the fiberglass cool enough that you can still touch it.

Clean and dry both surfaces. Wipe with the solvent named on the label (often alcohol or acetone; test on gelcoat first). Mask around the fitting with tape so squeeze-out is easy to clean. On a cored deck, the holes need to be sealed before bedding; see the deck hardware rebedding guide.

Use enough sealant that some squeezes out all the way around the base and into the bolt holes. Put a ring around each bolt under the head and around the hole. Starved joints are the most common cause of leaks.

Butyl tape is different: roll small strands and wrap them around each bolt just under the head, then make a fat cone of tape where the bolt leaves the base, and cover the underside of the fitting. In cool weather, warm the tape with a hair dryer so it's workable. BoatUS adds three cautions: use it only on bolted hardware (turning a screw breaks the seal), skip black butyl because it stains, and expect to find the right grade at an RV store or a marine-specific supplier rather than a hardware store.

With curing sealants, don't crank the fasteners down right away and squeeze it all out. BoatUS explains that a paper-thin layer, especially of polysulfide, can tear with tiny movements. A common method is to snug the nuts so sealant oozes out, let it begin to cure, then tighten the rest of the way. Turn the nut from below, not the bolt head, so you don't break the seal. Butyl tape creeps slowly, so it often needs re-tightening several times over a few days.

Cut the excess away with a plastic blade once it has skinned or cured enough to hold its shape (timing depends on the product). Then pull the masking tape. Clean uncured sealant from tools with the solvent on the label. For butyl, peel away the extra; BoatUS suggests a cleaner wax rather than mineral spirits near the joint. Trim so only a thin edge of sealant shows under the fitting; Don Casey warns against leaving a raised fillet, because silicone collects dirt, polyurethane yellows and polysulfide weakens in the sun.

Cure times vary widely and slow down in cool, dry air. 3M lists full cure for regular 5200 as up to about 4 days, while its Fast Cure 4200 and 4000UV products typically cure in about 24 hours. Polysulfide is slower. Don't load a fitting or launch over a fresh below-waterline seal before the label says it's ready.

Practical Sailor's adhesion test of 14 common caulks adds some useful real-world notes:

• Polyurethanes cure with moisture from the air. In dry weather you can speed them up by raising the humidity, for example a damp towel or a bowl of water under a small tarp tent over the joint, or a light mist after the first 12 hours.
• A skin isn't a cure. Several products skinned over in hours but were still soft inside weeks later, especially on metal and plastic, which don't hold moisture the way fiberglass and wood do.
• For high-load joints, wait longer. Most products can go into normal service within the maker's stated time, but the testers suggest waiting about a month, if you can, before a bond takes heavy loads.
• Polyethylene (StarBoard-type plastic) barely bonds to anything. Only one product in the test bonded to it at all, so fasten polyethylene parts mechanically and use sealant only to keep water out.

Look for water trails, weeping around bolts or a damp spot below. Check below-waterline fittings right after launch and again before you leave the boat. Log the job in BoatMend with the product used, so whoever removes it later knows what they're dealing with.