1942 Century Imperial Sportsman: Why Filling Seams with 3M5200 Is Forbidden

This morning’s 1942 Century Imperial Sportsman preservation project update is a plea, “Please, please do not pay 3M5200 into below-waterline seams.”

It is an adhesive, not caulk, pure and simple.

I have lost count of the number of otherwise wonderful woodies who arrive at the shop presenting curtains of old 3M5200 – stalactites – hanging from the bottom plank seams.

Once cured, 3M5200 will not compress when wetted planks try to expand. The result? Here is stark evidence of what happens next. The planks buckle and split, as several of them have in the ’42 Imperial Sportsman.

I have begun cleaning the seams using a curved pick and a reefing hook, supported by utterances that are for other than polite company.

As John noticed when he examined my growing pile of released 5200, “Most of this stuff never even adhered to the edges of the planks! Look at all the dirt and debris. So, not only did putting it in there buckle planks, it did not even work as an impediment to moisture infiltration!”

Right and right.

OK, what will we use in its place? I am opening seams that are up to 3/16” wide, which is at the outer limit of what it can bridge effectively, but we will use Interlux Seam Sealer for below-waterline applications. It cures, holds paint, and makes a watertight seal, but, even when cured, it never becomes hard. It compresses as the planks expand and expands if/when the planks shrink.

After applying the CPES and one coat of Interlux 2000E barrier coat primer, we will work up to three coats into the seams until they are almost fair with the planks. The wider seams may require more.

Some folks use Life Caulk for this purpose, a product I have used for bedding sailboat deck hardware, but never below the waterline, so I really do not have a view on its efficacy or appropriateness for this purpose.)

Then we will apply four more coats of the 2000E before we apply one coat of Pettit Tie Coat Primer and at least three of the deep, dark green topcoat our owner has approved. (Since she will be a trailer boat, We will likely reach for a paint like Interlux Brightside or Premium Yacht Enamel, unless we can find a gloss hard bottom paint.)

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