The 9 Best Automotive Seam Sealers

The 3M Urethane 08367 (about $33) is a one-part, moisture-curing urethane seam sealer in a 310-milliliter (10.5-ounce) cartridge that fits any standard caulking gun. It's thick enough to fill wide gaps, and it can be smoothed to a thin edge or brushed for a factory-style texture. 3M lists a tack-free time of 25 to 40 minutes, paint after 30 to 45 minutes and a full cure in about a day. The same formula comes in gray, beige and white, so you can match what's already on the car. The label marks it for industrial or occupational use, and 3M's safety data sheet for this family lists a small amount of isocyanate, so wear nitrile gloves and work with good airflow. 3M also advises laying seam sealer over primer rather than bare steel.

The SEM 29392 (about $33) is a one-part seam sealer in a 10.1-ounce cartridge, made in the U.S. SEM sells it in black, white, gray and beige to match original-equipment seams, and it says the sealer can go straight onto bare metal and can be resistance welded through. The can lists a working time of 5 to 10 minutes and a paint window of zero to 48 hours, so you can top coat right away or the next day. SEM's blog gives a 24-hour cure for its one-part line, so read the data sheet for the can you get. That short working time means tooling one seam at a time. Owners report good results on restorations and even an old trailer, though some say the cartridge came without a nozzle tip.

The 3M Dynatron 550 (about $31) is a gray seam sealer in a 12-ounce cartridge that you lay down with a caulking gun and then brush out, a good match for the brushed seams many cars get at the factory. 3M says it skins over in 10 to 15 minutes and cures fully in about 24 hours, and it holds its shape on vertical seams without sagging. It's a low-cost way into a name brand. Published paint times range from half an hour to a full day depending on the seller, so check 3M's data sheet before you spray. NAPA lists its VOC content at 376 grams per liter, more than three times the figure for 3M's urethane sealer, and some owners mention a strong smell, a stringy texture and trouble smoothing it in the cold. It's labeled for industrial or occupational use.

The Eastwood 51655Z (about $37) is a black high-solids seam sealer in a 10.1-ounce (300-milliliter) cartridge for a standard caulking gun. Eastwood aims it at the jobs restorers do most: firewalls, floor pans, trunk seams, fender extensions and taillight openings, and it says the sealer insulates as well as keeps water out. The label says it can be painted in 30 minutes, though some retailers say up to an hour. It stays rubbery once cured, which Mustang and Bronco owners like on floors that flex, but Eastwood notes it doesn't sand well, so shape the bead before it skins. A gray version is sold as 51656Z. Eastwood publishes fewer technical details than the big paint-shop brands, so expect less guidance on cure times and primers.

The Evercoat 365 (about $57) is a brushable seam sealer in a quart can. Evercoat now lists it as 100365, while many stores still call it 365. It goes on with a stiff brush, which is the way to copy the brush-stroke seams found on many original floors, trunks and door jambs. It's meant for spot-welded, riveted and bolted seams on bare or painted steel, aluminum and fiberglass. Retailers list a skin time of about 10 to 15 minutes and paint after about an hour, but those figures vary, so check the can. Owners say it spreads smoothly and stretches a long way, though a quart costs more up front than a cartridge, and a brushed coat can't fill a wide gap the way a gunned bead can.

The 3M 08308 (about $54) heavy-bodied seam sealer is a two-part epoxy in a 200-milliliter (6.75-ounce) dual cartridge that meters and mixes the two parts as it dispenses, so there's no hand mixing. The thick paste doesn't flow, which lets you tool it into the shaped, ridged beads seen on many factory seams, and it stays put on vertical joints. You'll need a two-component cartridge gun and mixing nozzles, which aren't part of the price. Epoxy is stiffer than urethane, so it suits weld flanges better than seams that flex a lot. Like other 3M body-shop products, it's labeled for industrial or occupational use only, so read the safety data sheet first. A self-leveling version, 08307, is sold for flat seams.

The SEM Dual-Mix 39387 (about $56) is a two-part, self-leveling seam sealer in a 7-ounce dual cartridge. It flows like a thick liquid, so it fills crevices and lays flat in roof-skin ditches, drip rails and floor seams that sit level. SEM says it can go on bare or primed metal and won't shrink or crack, and it recommends it for areas that don't flex much, such as roof skins. The label lists a 10-minute working time and says it can be top coated after 30 minutes, while SEM's product page says 15. Because it runs, it won't stay on a vertical joint, so this is a sealer for horizontal work. It also needs a two-part cartridge gun and mixing nozzles.

The ACDelco 10-2013 (about $18) is GM's original-equipment medium-body seam sealer (GM part 12378500), the kind a dealer body shop reaches for on factory joints. It comes in a 10.1-ounce cartridge for a standard caulking gun, works on vertical and horizontal joints without sagging, and ACDelco says it stays flexible without shrinking or cracking. It skins quickly and can be painted after 60 minutes, and the label says excess wipes up with mineral spirits before it cures. It's one of the least expensive name-brand sealers sold. The label also says it's for professional use and not for retail sale, and it lists flammable solvents including toluene, so ventilate the shop and wear gloves.

The Transtar 4167 (about $27) Ultra Flex is a black one-part seam sealer in an 11-ounce cartridge from a brand that sells mainly to collision shops. Transtar's data sheet gives a 3-minute working time and paint after 30 minutes, and says it's made to flex with unibody panels. Its VOC content is listed at 2.7 pounds per gallon. A beige version is sold as 4157, and some store listings mix the two numbers up, so check the color before you buy. Three minutes is very little time to tool a seam, so work in short runs and smooth it with a gloved finger dipped in soapy water. Owners say it dries fast and cleans up with little fuss, though reports on adhesion are mixed.