Chandlery March 2005 Issue

Map Seal

A safe, economical way to preserve charts and other documents on board without much fuss.

Last year, when Practical Sailor tested about a dozen spray products for rendering woven fabrics water-repellent (see PS June 2004, "Water Repellents for Fabric"), a sample of Map Seal® was submitted. Unlike the fabric sprays, which are supposed to let the material "breathe," Map Seal dries as a solid film that is meant to waterproof paper. It is described by its maker, Trondak, Inc., as a polymer coating that protects from water damage topographical maps, nautical charts, lab documents, recipe cards—essentially anything made of paper.

Another important benefit of Map Seal: The clear flexible coating means that the paper can be folded and unfolded countless times with minimum damage. The polymer acts as a plastic hinge. Scuba divers, kayakers, and small-boat sailors will all like that.

We found Map Seal on line for $5.99 for an 8-oz. bottle (at; it's available mostly at outdoor gear stores.

Map Seal is meant to be applied much like a varnish or paint. Just place the chart on a flat surface and apply a thin even coat with either a foam or bristle brush. Work fast because, although Map Seal is waterbased (and easy to clean up, when wet, if you work quickly), there's a lot of evaporant in the stuff. Working slowly could mean you'll develop ridges. Hang the chart up to dry for about five minutes (clothes pins, the kind with a spring, work well). Then take it down and recoat (the surface will still be tacky). Dry for 30 minutes and repeat on the other side. The finished job needs 24 hours to cure. And the company claims it's "non-yellowing."

Shown at the top is the backside of the chart after the initial coatings on the front. The degree to which the Map Seal, (the square and the triangle) differs from the 303 (the circle) is indicated by the soak-through that occurred with but one side coated. The water-based Map Seal just barely wet through the paper in a few spots and changed the chart paper appearance very little. The 303, with a much stronger solvent, quickly saturated the paper, and seemed to make it somewhat transparent. Above, this photo shows the thoroughly dried chart after being soaked, dried, folded roughly 50 times, and again wet down overnight. The object was to see if the folded seams had failed, which might permit water to penetrate by osmosis. Although the untreated portion was soaked and there was some darkening within the 303 circle, both the square and the triangle, treated with Map Seal, appeared unchanged.

It's easy to use, and comes in just about the consistency of soapy water. Apply sparingly because it runs freely. Applying multiple coats, two on each side, with drying time in between, requires nearly an hour. An 8-oz. jar is said to be enough for 8 to 10 charts; a claim that we doubted until we put it to the test. Being like water, it goes a long way.

To test the Map Seal, PS used a portion of a favorite local chart—a No. 13218, Martha's Vineyard to Block Island, on which we've plotted many a New England cruise.

A four-inch square in the waters off the southwest tip of Martha’s Vineyard (see photo) was coated on both sides with Map Seal.

Then, in defiance of the instructions, we coated a four-inch equilateral triangle on but one side. Because the second coat on the first side of both the square and the triangle soaked through the chart paper in a few spots, the question arose about whether both sides need coating.

Then, to cross-check a good fabric spray against the Map Seal, a four-inch circle was coated on both sides with 303, a water repellent spray that was the best performer in the fabric spray test mentioned above. The first coat of the 303 soaked through to the back of the chart quickly. Map Seal is about a buck an ounce and 303 is 73¢ an ounce; not much price differential.

We found that it's easier to brush on the Map Seal, just wetting the paper, than it is to spray the 303, which seems to go on too heavily.

After drying, it was noted that the Map Seal shapes were slightly wavy, which might be annoying if you do finely-detailed chart work. The 303 shape dried very flat and looked like the uncoated paper.

Both the Map Seal shapes, the 303 circle, and the untreated portions of the chart accepted pencil work and erasure equally well.

The Testing
The NOAA chart sample was placed outdoors for several drizzly days. The chart was turned over several times, mostly to make sure the triangle with only one side coated got a good, all-natu;ral wetting. When retrieved, the uncoated portion of the chart was sodden, rough, crumbly and structurally weak.

The circle (treated with 303 fabric spray) was slightly darkened, suggesting that some moisture migrated into the paper, under the coating. The surfaces, both front and back, felt smooth—like plastic.

The square (double coated with Map Seal) and the triangle (one side with Map Seal) appeared dry, and the paper was strong. Excess moisture could be wiped away; a misting with water resulted in beading on both figures.

After the sample was dried for several days, we sprayed water on the uncoated areas and it quickly soaked in worse than ever. However, the water beaded fairly well on the 303-coated circle and very well on the Map Seal rectangle and triangle.

The sample then was folded and unfolded, back and forth, about 50 times. The uncoated areas weakened and tore easily, but the coated paper in the geometric figures seemed strong as ever.

For a final test, the entire sample was again wetted and left to soak overnight. The stressed seams in the coated areas did not appear to admit moisture, either directly or by osmosis. The uncoated area was like a sponge.

The Bottom Line
So why pay $18.25, as PS did, for an NOAA chart and spend an hour waterproofing it—especially if you've started converting to electronic chartwork? If it's waterproof charts you want, you can buy them for about $20. Some have two charts, one on each side. Large-print charts go for $23. However, not all charts are available in waterproof versions.

So, you may want to seal and protect: (1) a few harbor charts, (2) your favorite charts of local waters, (3) large-scale charts for voyage planning or off-shore navigation, (4) Canadian or other foreign charts, (5) or maybe even every page in an entire chart kit book, for which you've probably put up $120. (If you preserve a chart kit, don't let the wet surfaces dry together; Map Seal is sticky.)

For sailors, the bonus of using Map Seal is that anything treated with it can be folded and unfolded many times, back and forth, inside and out, without that annoying failure of the seams.

Map Seal is inexpensive, and it works somewhat better than the fabric spray we tested. It can be handy for coating any papers you want to last—in the marine environment or anywhere.

Contact - Map Seal, 360/794-8250,

Comments (0)

Be the first to comment on this post using the section below.

New to Practical Sailor?
Register for Free!

Already Registered?
Log In