Ron, here's a step-by-step guide and a couple of pics from my DIY replacement of a latex neck seal on my trilaminate drysuit (DUI CLx450). There are other methods, but this works.
This may generate more questions than answers, but hopefully it's a good start. Just give a holler if it's unclear. I have more pics, if needed.
Latex neck seal replacement on a trilaminate drysuit
1. Peel off the old latex seal. For the most part, mine peeled off fairly easily with just firm, steady pulling, but it sometimes came off in pieces and sometimes required judicious use of a heat gun. I've heard that a clothes iron will also help to soften the glue's grip.
2. Roughen and level the surface of the old glue. I used a Dremel coarse sanding drum and took most the old glue off, but just roughening and leveling is probably fine. Wipe the glue surface clean with a rag dampened with toluene. Lacquer thinner works nicely, too.
3. Make a round template disk that will be used to fully spread and anchor the neck opening of the suit. The template should be sized or marked to match or slightly exceed the diameter of the contact surface of the new latex neck seal. I used 1/4" polycarbonate cut into a circular disk and marked it with concentric circles as guidelines for the inner edge of the suit opening. You can even use stiff cardboard for templates.
4. Make a second round template for the seal itself, this one cut as a ring-shaped disk to match the size of the contact surface of the latex neck seal.
5. Roughen the contact surface of the latex seal with sand paper and wipe down with toluene.
The picture below shows the two templates and the sanded surface of the old glue on the suit neck opening.
6. Apply double-sided tape to the suit disk along the circle where the suit material should be anchored. I made a continuous circle with the tape to improve its grip on the porous trilaminate material of the suit. Insert the suit disk into the suit opening and fully spread the suit material perfectly round and flat before pressing it onto the tape to anchor it.
7. Apply double-sided tape to the ring-shaped disk. In my case, I applied the tape radially every couple of inches as can be seen through the clear polycarbonate in the picture below. Use more tape if needed to hold the latex securely and flat.
8. Position the flange of the latex seal over the seal disk and press it onto the tape to anchor it flat and round.
9. Apply contact cement to the intended areas of contact of both the suit material and the latex seal. I used an industrial 1-part neoprene cement called S-18, which is similar to ordinary contact cement, but stronger. (One of its uses is to glue marine carpet onto boat hulls, but it's the preferred glue of a local drysuit repair shop in my area.) I find it best to use 3 moderate coats of glue with about 10 minutes of drying time between coats or until tacky.
10. Lay down thin spacers on the tacky glue of the suit opening to prevent contact of the two glued surfaces during positioning. (I used flat wooden sticks about 1/16" thick).
11. Once the templates are positioned to accurately align the intended contact areas, begin to remove the spacers one at a time, each time pressing down the top template to make the glued surfaces come in contact where the spacer had been. See pic below.
12. Once all the spacers have been removed, again press the template firmly to improve the contact of the glue.
13. Remove the seal disk from the latex.
14. Roll the latex firmly a few times to further improve the contact of the glued surfaces. I used a wallpaper seam roller. If you've been careful, you won't have any significant bubbles trapped. If there are bubbles, they can usually be rolled out or left in place if they're very small.
15. Remove the other template from the suit.
16. Wipe any residue of tape off the suit and seal. Neutralize any overspill of contact cement with unscented talc powder or corn starch.
17. Seal the inside edge of the latex with Aquaseal (thinned about 50/50 with Cotol or the nearly equivalent toluene). A thin, flat bead about half an inch wide is fine, centered over the edge of the latex.
As I said, this doesn't cover all the details so just ask if something needs further explanation.
If anyone sees anything I've missed or has better ideas, feel free to chime in.
HTH.
Dave C