I went away for a week and my dad got started on the job. He gave me the following detailed assessment of the work, which I thought might be useful to anyone attempting this in the future.
The work has gone really well so far but it has turned out to be more time consuming than I had anticipated. I know that is partly due to me - lack of experience with this task as well as some perfectionist tendencies.
1) The rubrail system is somewhat different from what I had anticipated and described above and I had to make adaptations. The system is not technically complicated so that wasn't an issue. One of the differences is that the black parts are not inset into the hull they are shaped to hold the stainless steel trim embedded and they have a protruding lip on the underside that is obviously designed to provide a watertight seal from waves and splash while the boat is running. They are made of hard black plastic (not rubber) and are very sturdy. I found all of them in good condition and nothing needed replacement except the stainless steel screws. I wanted to change all of those even though the originals were still in acceptable condition.
2) The black plastic strips are not only held in place by the visible stainless steel screws (which fasten the stainless steel trim to the boat) they are also fastened at about six foot intervals with aluminum pop rivets. These were hidden under the stainless trim. I decided to carefully work one of the rivets to see if I could pry it out without damaging the fibreglass hull or the black plastic trim. It turned out that the soft metal of the rivets could be pried away from the hull about half an inch. Once pulled out a bit, most of the rivets just snapped in half and a few actually popped out of the hull without causing damage to the glass or the trim. To remount the trim, I used a countersunk stainless steel screw of the same dimensions as the rivets. The threads of the screws bite into the fibreglass really nicely and in my unprofessional opinion that makes them just as good if not better than the rivets. The aluminum was so soft and easily broken that it took me a while to figure out why the builder would even have used them. I suspect that the only reason to use the rivets was convenience. Since the screws that fasten through both the stainless trim and the black plastic into the hull at six inch intervals must serve as the main anchors for everything, the only real purpose of the rivets was to hold the plastic in place until the screws were set in. here is another set of the same rivets that appear to join the upper and lower parts of the hull under the black plastic trim. These are located at intervals of about five feet in spacing and as far as I could tell are there to stabilize the seam until the stainless steel trim screws are set into place. There was no good reason for me to remove those rivets since I could do all of the required removal of the old caulking and installation of the new sealant without touching them. I was fearful that if I took even one of those rivets out, it might cause the two sections of the hull to shift ever so slightly and then nothing would fasten back in properly. In hindsight, I am really happy to have made that choice.
3) Once the trim pieces were removed, the biggest and most time consuming part of the job began. I spent hours and hours carefully clearing out the old sealant and debris that had accumulated between the upper and lower sections of the hull. That was just plain tedious but absolutely essential. Once most of the sealant was cajoled out of the seams, I then used my Olfa knife to clean any remaining bits from the surfaces. Then I went over the entire area first with Goof Off (an acetone based cleaner) and then pure Acetone. I had to keep rubbing and rubbing in most of the places where the old sealant was still adhering because it was essential to have the new sealant locking onto the hull cleanly and without any gaps through which water might enter. I finally got it to the point where it looked really clean and free from debris.
4) The 4200 sealant is excellent and once I could finally start to apply it, I discovered that it is quite different than the bathroom and kitchen caulking I am used to. Because it is acrylic and solvent based, it is like working with glue. I used a thin bead just to make certain that it wedged as far as possible up into the seam between the upper and lower parts of the hull. That took more time and effort than applying a thicker stream and required some heavy duty pulling on the caulking gun trigger but it worked beautifully. Along much of the hull, I could actually see the caulking starting to ooze out through the screw holes so it was obvious that it was penetrating nicely.
There were no gaps anywhere so the material obviously does an excellent job of sealing the seams. BUT it is a total bear to get off your hands, tools, and any other places it comes into contact with! So I ended up spending a few additional hours with acetone carefully cleaning all the surfaces where it contacted the hull in spots that wouldn't look as pretty with caulking smears. As for my work gloves, hat, etc. well they are expendable fortunately and I have thrown out most of the work cloths that I used for the wiping and cleaning. For my hands, I expect that when the old skin sloughs off eventually the caulking will disappear too. It is amazingly sticky stuff.
5) The 4200 is perfect for the hull pieces but was not going to work for remounting the trim pieces (black plastic and stainless steel). The problem would be that the white would be visible in spots where you would not want to see it. So I wandered over to the marine shop and bought a tube of the equivalent material in clear. I used that to seal the screw holes. I still want to apply a very thin bead along the edge where the black plastic meets the hull. As described in Step 9 above, I applied a thin bead on the inside edge of the black plastic (top and bottom) but I don't feel that this is quite enough to absolutely seal any gaps. That application is going to take time but I feel it is worthwhile. It was obviously not fully sealed previously because there was moisture and bits of sand and other debris between the black trim and the hull when I removed it. I feel that some extra effort to ensure that any possible gaps are fully sealed is worth the time.
There is also a spot where two ends of the black plastic are not fully aligned and even though I spent time to try to get them exactly matched (they weren't in complete alignment before I removed the pieces). Although they may be watertight now, I am still not happy with the appearance so want to go over that part again.
6) Screw mounting. Another unexpected feature I discovered along the way is that there is not a single type of stainless steel trim screw. There are actually three different types. I was able to get a common oval Robertson head for each of the three different types so when installed they all look the same which is nice. They each serve a different and important purpose but that again was not covered or described in the twelve step process. Two of the types of screws are quite a challenge for one person to remove and install. There is a distinct advantage to having one person inside the cabin holding tools and moulding while the other does the fastening outside. Overall the screws have seated solidly and fastened into place very well. I am confident that the work thus far is completely watertight and will stay that way.