Seam sealer or paint first?

[57 Handyman]

Couple of thoughts: our classic cars are old, so the original factory sealers will dry, crack, and leak...think about it! Just because 35+ years later, we find these issues doesn't make the product a failure. When I did extensive repairs and replacement of the lower body panels, after spot welding these multi-layers to form seams, I applied direct-to-metal body sealer before applying primer and undercoat. The direct-to-metal product I used was recommended by the GM of a local body repair shop.

Fusor® 800DTM/801DTM/803DTM Direct-to-Metal Sealer

800DTM, 803DTM, Fusor sealer/adhesive, direct to metal sealer, direct to metal adhesive, direct-to-metal, UV resistant, Fusor 800DTM sealer/adhesive, Fusor 803DTM, sealer/adhesive, 800, 803

www.lord.com

 

[Bonnewagon]

That is a serious product! Even weld through? Looks like only industry body/paint supply places carry it. I have a guy.

 

[57 Handyman]

You're correct, I did go to the supplier to my friend's shop. Also, this product isn't cheap. Product has a 15 month self life so be sure you get and use it in timely manner.

 

[08Malibu]

Most seam sealers including 2k epoxy sealants work best applied over painted metal. Most seam sealers need to be painted afterward to UV protect them.

That’s not true at all. There are some sealers that work direct to metal. The best way would be epoxy, then a 2k seam sealer, then paint. You never want to seam seal over 1k primers or paint, or etch primers.

 

[Clone TIE Pilot]

That’s not true at all. There are some sealers that work direct to metal. The best way would be epoxy, then a 2k seam sealer, then paint. You never want to seam seal over 1k primers or paint, or etch primers.

I only use 2K primers so I agree with you. I assumed anyone going through the trouble and cost of using quality seam sealer would use 2k epoxy primers. Even most 2k sealers that say direct to metal still recommend epoxy primer first. Most seam sealers are not designed to protect bare metal from rust.

I had 2k sealer degrade from UV damage if I only painted over it. I found if I applied another coat of 2k epoxy primer over epoxy seam sealer, then paint the sealant would last.

Overall, its best to read and follow the instructions of the products you are using.

Moreover, coatings such as POR15 are designed to be applied over rusty metal that is treated with rust conversion primer in order to adhere, not clean metal. If applied over clean metal such coatings will peel off in sheets. Even direct to metal seam sealers require clean bare metal and do not work on rusty metal.

 

[Bonnewagon]

I think I will use the Rustlok first. I have been using it for many years on my boat trailer. It does not yellow from UV exposure. It contains aluminum flakes for UV protection and I feel it is stronger than POR-15. I have had good results with the POR but it still can scratch and chip. The Rustlok I used on my trailer sits outdoors all year long and gets dunked in salt water. It is physically hard to scratch. While both are moisture cured, the Rustlok can be applied in cold temperature. Both are thick enough to fill holes and gaps in the welds. But I doubt either can be expected to survive much metal flexing. That is where the seam sealer comes in. I guess if I apply it while the paint is tacky it will adhere the best. See this trailer- it is 34 years old. The paint is what used to be called TrailerCoat- now called Rustlok but is the same exact formula according to Pettit. .

 

[08Malibu]

I think I will use the Rustlok first. I have been using it for many years on my boat trailer. It does not yellow from UV exposure. It contains aluminum flakes for UV protection and I feel it is stronger than POR-15. I have had good results with the POR but it still can scratch and chip. The Rustlok I used on my trailer sits outdoors all year long and gets dunked in salt water. It is physically hard to scratch. While both are moisture cured, the Rustlok can be applied in cold temperature. Both are thick enough to fill holes and gaps in the welds. But I doubt either can be expected to survive much metal flexing. That is where the seam sealer comes in. I guess if I apply it while the paint is tacky it will adhere the best. See this trailer- it is 34 years old. The paint is what used to be called TrailerCoat- now called Rustlok but is the same exact formula according to Pettit. . View attachment 213154

If you let it dry and scuff, you’ll probably be better off. I’ve noticed 1k seam sealers work better over 1k finishes, than 2k ones do.

 

[Bonnewagon]

let it dry and scuff,

I don't know. It is some very hard stuff. Maybe an orbital sander would work. But with both being urethane would they not be chemically compatible? The sealer says paintable in an hour but the Rustlok warns that if it is hardened you will regret it.

 

[Clone TIE Pilot]

I don't know. It is some very hard stuff. Maybe an orbital sander would work. But with both being urethane would they not be chemically compatible? The sealer says paintable in an hour but the Rustlok warns that if it is hardened you will regret it.

In that case you may want to apply sealer during the recoat window. Those over rust coatings are mainly intended tor frames, not so much for autobodies that require sealing. They are also a shortcut for proper cleaning and repainting.

 

[Bonnewagon]

What I usually do whenever all the carpets are out is to apply a coat of POR-15 to the entire floorboards, trunk, wherever water can cause mischief. Because if it is not leaking now it probably will down the line. This way I am protected for when the inevitable occurs. As we all know leaks are never obvious at first. They begin small and the carpets get wet, hold onto it, and the rust begins. We find it out too late so a coat of rustproofing when the metal is sound is just good insurance.