Sewing Waterproof seams: Sealant or seal tape?

Sewing Waterproof seams: Sealant or seal tape?

Why do you need to seal stitch lines and seams?

Waterproofing seams and stitch lines ensures your handmade waterproof jacket is just that: waterproof. Every time your needle punctures the surface of your waterproof or water resistant fabric it leaves a little hole, even when plugged with the thread it’s an easy way for the water to creep in. Also, as you can see from the picture below, when you construct a garment by sewing two pieces together, there are gaps created between where the pieces are connected by the stitching.

A glimpse of light through the seams sewn between two components of waterproof fabric.

I’ve made and owned water resistant gear in the past that doesn’t have sealed seams, and they work … well, just fine. In light showers, for short walks, the water is repelled. But for longer journeys and wetter conditions you’ll want some certainty that you will stay snug and dry. In this case you’ll want to seal the seams as you sew your garment using iron-on tape, or brush-on sealant from the tube.

If you want to make a raincoat, poncho or jacket that keeps you snug and ensures you stay dry, you’ll want to seal the seams.

What is better: Seal tape or sealer from the tube?

There are two main methods for sealing seams on homemade raincoats and wet weather gear. Iron-on sealant tape, and liquid sealer. I snapped these both up on eBay for around £10 each.

Roll of iron-on seal tape and a tube of seam sealant liquid.

Testing iron-on seam sealer clothing tape

5m of light grey iron-on sealer tape, 20mm wide, set me back £10 on eBay. I don’t recall seeing colour options so I grabbed the grey roll. Later on I spotted a few different monochromatic shades. I probably would have purchased the darker grey or black for this garment if I’d taken the time to look around.

How it works

After each piece of fabric is stitched together the seam allowance is pressed flat and the tape is ironed to the inside. The tape has two sides, a textured top side and thin sealer underside. As the tape heats up the side of the tape with sealer on it melts and becomes tacky, in its melted state it adheres to the seam, and as it cools and hardens it cements the bond. This finishes the seams and neatly seals the gaps created by contraction.

Tips for using sealant tape

1. Watch your heat

Pay attention the heat of the iron to find a sweat spot that’s warm enough to melt the taps but not distort the fabric. I did a bunch of test pieces and noted the best iron setting. You’re only a hasty iron press away from melting your pieces, so it pays to take it slow.

2. Turning corners

You can apply the tape around a light curve. I’ve not yet tried. A sharp angle but you might want to try cutting notches to follow a sharper corner.

3. Conserve your tape

The instructions said to leave a bit of overhang over the edge of the seam and then trim. But I wasn’t too keen to bulk out the seam allowance and waste my precious tape. In the end I worked by ironing on the loose end, with the rest of the roll on the ironing board or in my pocket. That way I could trim it from the roll half a cm from the edge.

Testing a tube of seam sealant

The tube of sealant was cheaper than the tape, at £7.99 for 60ml. Which I guess would go quite a long way. Again, like with the tape, I played around with a load of test pieces before using it for realsies. Straight away the strong chemical odour and messy runny mixture put me off. Fiddling about removing the lid and putting on the spout did not fill me with joy. It’s tricky to manage the amount of sealant applied. The instructions say to apply the sealant and then brush it into the seam with the little scratchy brush. This worked ok when I got the knack, but I noticed that the liquid and the bushing action did cause the edges of the pieces to curl up.

Tips for using liquid sealant

1. Manage the mess

Lay down paper or newspaper to protect your workspace from escaping sealant.

2. Ventilate your space

If, like me, you can’t apply it outdoors, crack a window. Sealant has a strong, unpleasant, chemically odour.

Results: Iron-on tape or liquid seam sealant?

For my task of sealing a sew-at-home raincoat, hands down the iron-on tape won across the board. I love how it was easy to work into my construction flow as part of the seam pressing step. The seam allowances sits neatly and aids future construction steps. It’s not messy and doesn’t have a very intrusive odour.