Silversailor,
When cleaning teak you should always rub across the grain not along or against the grain. Rubbing along or against the grain will quickly cause the softer growth to be eroded away. Teak is a "soft" hardwood and it is easy to inadvertently erode.
When you say "some of the material around the seams is disappearing" I gather you mean the teak not the Sika? If it is the teak the only thing you can do is sand the teak/Sika down to give you a new flat surface. But the teak is thin (about 7mm) on most boats these day so you don't have much teak to sand. But you should be able to sand it a few times before there is a problem. When you sand teak you should sand along the grain not across. If you sand across it is very difficult to get the sanding scratches out. For such a small area I would hand sand using a cork block to wrap the sand paper around or you can get plastic "floats" with a handle that you can attached the sand paper onto which are larger than the normal cork blocks. You need to keep the block flat on the seat when sanding so you get a flat finish. Don't sand with the sand paper only in your hand as the uneven pressure applied by your fingers will give you grooves in the teak. Use a 60 or 80 grit sand paper. Any thing finer and you will be at it all day.
If it is the Sika which is "disappearing" the seams can be recorked. The solution is to take out the old Sika runs and reapply the Sika. Carefully cut either side of the Sika joint with a sharp boxcutter and then you should able to pull the Sika out. To reapply - mask the joint on either side with masking tape and apply the Sika into the groove leaving it slightly "proud" because as the Sika sets it shrinks so if it isn't "proud" you will end up with a joint which is concave. You can smooth the Sika run by running your finger along the joint. You should ware latex type gloves when working with Sika. Wet the gloves finger with water with a small amount of dish washing liquid mixed in and lightly run your finger along the run. The water stops the glove from sticking to the Sika. Remove the masking tape once the Sika has started to harden - say 20-30 minutes after application. Allow the Sika 24-48 hours to harden before sanding. It can be a messy job to reSika but once you get the hang of it, it is fairly easy. From memory the correct Sika to us is 290DC (deck corking) and make sure the tubes you buy have plenty of "use by" time left. Old or out of date Sika is harder to use and the job with be more difficult and may not last as well.
------------- Wayne W Cruising, currently in the Caribbean and will head across the Pacific early 2024
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