Tim Moyer is the harp tech behind Working Man's Harps, which means that he builds some of the best modified diatonics on the planet. Tim recently shared his techniques for sealing harmonicas with Micropore tape with the Harp-L list; we're reprinting Tim's comments (with a few of our own) here.
I've probably sealed as many harps with Micropore as all the other folks on Harp-L put together, so I feel qualified to tell you how. I
know there are a number of different ways to do it, depending on harp model
and personal preferences, so I'll just tell you how I do it. This is a
standard modification I do on harps that I customize, and it can greatly
improve the airtightness of a harp. (Richard Hunter says: it sure does. It's amazing how much this modification improves the airtightness of even a well-made harp, such as a Lee Oskar.)
I dissassemble the harp, and apply a layer of 3M Micropore tape to the inside surfaces of both reed plates where they contact the comb. (Note: Micropore is a trademark of 3M Corp., and you can buy Micropore tape at almost any pharmacy; you'll find it with other medical supply tape. It's a lightweight tape with mild adhesive on one side.) I do this on most harps with plastic combs, and have done a few metal combed harps as well (Meisterklasses and Suzuki Promasters). I don't recommend it for wood combed harps. For most Hohner harps the one-inch width tape is nearly perfect in width, though on the MS harps it's a little too narrow, and I use the 2-inch tape for those, and for Lee Oskars as well. I trim off the excess (which I save to put elsewhere, like between the edge where the cover contacts the comb in the mouthpiece).
I line up one edge of the Micropore tape along the front edge of the reed plate and press it down so that it covers the entire surface of the plate on the side where it contacts the comb. On the blow plate, where the reeds are mounted on the inside surface, I apply the tape right over the reeds. I then use an X-acto knife to cut the tape around the reed slots and screw holes. (Note: the medium weight precision #2 X-acto is a good choice for this work. You can buy it at most craft and hardware stores for about $4; an extra set of blades will cost another $3 or so.) Cut close enough to the slots to ensure that there will be tape between the plate and the comb; if you cut too much out, the comb will make uneven contact with the Micropore tape, and you may get air leaks as a result. On some models of harp you also need to trim around holes stamped for registration tabs and pins, as well as the standoffs for the covers on Lee Oskar and Hohner Special 20 harps.
Once you've cut around the reed slots, etc., just use the tip of the X-acto to lift the piece you are removing (be careful not to catch the reed) and carefully peel away the excess. Once it's trimmed I usually press it down again just to make sure I haven't pulled it up anywhere. Then reassemble the harp, adjust the reed gap and you should have a very nicely sealed harmonica!
Richard Hunter says: Thanks, Tim! This modification gives you more dynamic control over the harp at both the low and high ends of the dynamic range, i.e. it will allow you to play both softer and louder than you could before. (Don't forget to set the reed gaps for your style of playing, too.)
There's an alternative method for Micropore-ing that I use for Lee Oskar harmonicas, which is to apply the Micropore tape (2-inch) to the comb rather than the reed plates. Disassemble the harp, take the tape, and lay it glue side down on the comb; press it into place, making sure there are no ripples. Use the X-acto knife to cut out the tape in the reed slots, to poke holes in the tape where the screws go through, etc. I leave a little overlap on the mouthpiece side to ensure that there's solid contact between the reed plate and the comb all the way around. When you're done with both sides, reassemble the reed plates and cover plates, cut off the overlap on the mouthpiece, and you're in business. I find that it takes about 15-20 minutes to do a harp this way; it's something you can easily do while you talk on the phone.
Standard disclaimer: I have had no problems using Micropore tape on my instruments, nor to the best of my knowledge has anyone else. However, body chemistry is a very personal thing, and you may find that using Micropore in your harps is irritating to your mouth, lungs, etc. Neither I nor Tim takes responsibility for any problems you may experience in this regard; if you find that Micropore in your harps is causing you problems, stop using it and, if necessary, consult your doctor.