Don't use Sikaflex or any other PU adhesive! Good, that's saved you hours of pain later.
We supply two butyl based bedding products, one a tape (ARBO GZ) and the other what I will describe here as 'goo' (eg ARBOmast BR).
What makes a good bedding compound?
Something that will stay flexible over a long period of time (eg 20+ years) and be compliant enough to accommodate movement between the hull and the window frame. Then, when you want to remove the frame it's the compound that says 'fine I'll let go now' (in contrast to a PU adhesive which will say 'go on, try and fail sucker!'). Butyl may seem a little old school, but it does what it needs to rather well.
Tape or 'goo'?
Tape comes on a 12m roll and is 10 x 3mm in section. It is relatively clean to work with and particularly suited to frames with a narrow flange (eg most Lewmar portlights). Crucially, it is virtually impossible to squeeze down to less than 1mm thickness, meaning that try as you may you will not squeeze all the bedding compound out and thus may end up with a seal that stays the distance. It comes in white or black.
Goo is VERY messy if you let it. Often described (OK by me) as being like the cheese in a 1980's fondue party (eg Gruyère) it will string and hang onto anything it touches, whether intended of not. Be warned. It does a great job however when treated with respect. The main issue when using only goo is that all too often people just can't help themselves and overtighten the fasteners which in turn squeezes out all that lovely goo, leaving very little of anything between flange and boat. This allows leaks.
Using just tape
This is the cleanest option. Clean the mating surfaces with acetone or IPA first. It is likely the frames will have been cleaned with white spirit which leaves an oily residue. Without the acetone you may find the tape difficult to 'stick'.
Ideally, apply the tape around the frame flange. I suggest putting the join between the ends lengthwise but whether above or below is down to you. If you need to put screws through the tape punch the holes with a bradawl or similar. Do not use a twist drill as you will end up with tape wrapped around the drill bit.
The image below shows a Lewmar New Standard Mk1 frame (note the Mk1 staked on hinge mount) with tape applied. This is all you need for most of the Lewmar ports.
Using tape and goo
There are two options (shown in the images below) and are suited to wider flanges (20mm+).
Here, the GZ tape facilitates the minimum 1mm depth and the goo provides an extra level of flexibility. As above, clean the mating surfaces with Acetone or IPA prior to fitment.
Loose fit in cut-out - tape on outer edge
If a Friday afternoon cut-out you may find the frame rather loose in the hole. If so I suggest you use the tape on the outside and place the goo on the inside of the flange, between the tape and the bottom of the channel holding the glazing. Apply the tape first. The idea here is that the excess goo will squeeze into the void between the frame and cut-out. If the frame is very loose you may also want to pack out the gap on the lower edge (eg with chocking rubber) to help stopping the frame sink as a result of gravity.
The image below shows a Westerly Marsh Walters type frame with a flat flange, but with the tape on the outer side of the flange as discussed above.
Snug fit in cut-out - tape on inner edge
If you have a snug cut-out, then you can't have the tape on the outside as the excess goo will have nowhere to squeeze into, so put the tape on the inside. As this means the excess goo will be coming outward you need to do a little more anti-mess preparation.
First, dry-fit the frame and put at least two inches of masking tape around the periphery of the frame. This is a run off area for excess goo which can be removed later.
Next, remove and clean the mating surfaces as above. Then apply the tape to the inner side of the flange and remove the backing tape.
If using interscrews it is critical that you don't get goo in the captive bush part as once the machine screw in engaged it will have no exit. If the screws are going through the tape this is not a problem (though you may have to make the holes using the bradawl as above). If the screws go though the goo, you may want to put the screws in place before you apply the bead of goo. If so put some masking tape over the screw head to hold in place in the frame. Then be careful with the bead not to get goo on the end of the screws.
Now apply a good bead of goo to the exposed flange.
Remember that the goo is VERY stringy so don't give it a chance to touch you or any other part of the boat!
Next, carefully loose fit the window and start to engage the screws. When all screws are engaged you can then start the tightening process a little at a time trying to draw the frame down parallel to the surface.
As with the other tape options you can apply quite a lot of pressure as though the tape will squeeze down it is unlikely to end up less than 1mm thick.
Note that if the fasteners are self tapping screws, rather than interscrews (ie nut and bolt) you may have to push the frame down more so as not to strip the thread in the GRP. Alternatively, with screws just use goo, but be careful not to squeeze it all out!
The image below shows a the window above with the tape applied to the inside.
Cleaning goo - IMPORTANT!
If using the tape on the inside and goo on the outside of the flange the excess will spill out onto the masking tape around the window. RESIST the temptation to try and clean this. LEAVE IT for at least a day, preferably a week, until it has time to skin over as this will make it far easier to clear up.
After a week, return with a large box or bag (to transfer the tape/goo to). Using an acrylic scraper or similar tease away the excess from the flange edge onto the tape and then when done, carefully remove the tape and place in the box taking care not to allow any to touch your deck or hands. Then, use some white spirit on a cloth to clean any remaining traces of goo. If there are lumps of goo (eg around screw heads) remove the worst before using the cloth as white spirit on a blob will result in an even more messy blob!
Now go an have a beer and congratulate yourself on a job well done. No need to tighten the screws more later (this is not a PU adhesive after all).