I have used SikaFlex to seal hatches etc on my Hobies.
There are a number of different versions of this product e.g. "Sealant" "Adhesive" "Adhesive Sealant" & w.h.y. I think the version I found most effective was 291 but can't promise that I have the number right - either way I found the non-adhesive ("Sealant" only) version to be the least effective (more like conventional domestic silicone sealant than the other products in their line-up) and the ones with adhesive in their titles very good indeed. It comes in various colours: I have used black exclusively on my Hobies as it matches the hatch colour.
They advertise the product as Marine Sealant & I can vouch for the fact that once applied it does provide an incredibly resilient seal - it is flexible but tough (unlike silicon - which is quite soft), highly UV resistant and seems unaffected by salt (or fresh) water.
SikaFlex is NOT a gap sealer: you can't squirt into a gap like you can with silicon - you have to apply it in situations where you have a surface to be mated to another surface with the required seal between them (e.g. hatch flanges to deck cutout).
If you get it right it really does make a very good permanent weatherproof seal. It is expensive but personally I wouldn't use anything else as a marine sealant & have done all my Hobie twist & stow hatch-flange-to-deck joints with it.
Having said all that I now feel I must give a bit of additional advice based on personal experience before you go rushing out & goop up your pride & joy with it.
Be warned, that if you decide to use this product, application can get veeeery messy indeed if you are not very organised and methodical.
Until it sets Sikaflex is as goopy as silicon but incredibly tacky, sticks like the proverbial sh*t to a blanket and can only be cleaned up with a rag soaked in meths (aka wood alcohol). It has a tendency to get EVERYWHERE if you are not very careful.
I advise you to have a lot of unused clean-up rags and meths ready to hand before you start and somewhere to put down your goop gun where the drips from the goop nozzle can be caught; even then you need to be very careful to keep your hands clean as you go and not to leave any drips lying around (thus reducing the possibility of finding Sikaflex hand and foot prints everywhere later). THIS PRODUCT SHOULD NOT BE USED BY OR IN THE PRESENCE OF MINORS.
The best approach I have found is as follows:
Before you start:
- Put on your worst clothing.
- set up some kind of Sika-lock (similar to an airlock) between your work environment and the outside world - a Sika firewall is another way to think of it
- Practise making up the exact assembly you want several-many times beforehand so that you are 100% confident that you can put the components together right first time every time before you start - almost the worst thing you can do is to dis-assemble an un-set sikaflex coated joint.
- Be ready to clean up every small drop, smear, of raw product as you go, especially on your hands, feet, work tools and porous surfaces. Fence-off and leave large dollops to go off 80% before touching them unless they are going to mark a surface.
- Many people who are familiar with other brands will expect to be able to wipe excess goop away; I suggest you try not to do this as a first measure with Sikaflex - First put down your goop gun out of the way so that you don't accidentally bump into it/step on it etc. Now lift as much of the excess off as you can with any blade-tool that is suitable before wiping the spot and area clean with a meths soaked rag - or rags !
- Don't bother with plastic gloves - they are no match for this stuff !
When ready to go for it:
- Practise the assembly again, then
- Apply a bead big enough that you can be pretty confident of achieving a seal when assembled but not so much that you are going to have masses of it oozing out of the joints when you tighten the assembling bolts. Try to do this in one go/motion & try not to throw the stuff around more than you have to. Try to get it right first time because the last thing you want to do is to get too little on the components so that you have to take them apart to re-goop them.
- Assemble the components and part tighten them down which should get the stuff squeezing out of the joints - but not too much. At this point, so long as you have not really over-gooped the joints you should have a reasonably tight closure between the components (80-90%, perhaps) and some goop squeezing out of the gap between them as a bead along the gap.
- Now, AND THIS IS IMPORTANT, do not touch the excess goop (unless there really is loads in which case things are going to start to get messy) leave the goop to go off a bit so that it loses most of its stickiness, then clamp the joint up fully (more goop should squeeze out).
- Let the goop go off almost all the way (It must not be sticky and must be 'set' all the way through - if your first efforts to remove excess result in messy clean up you probably have not let it go off enough).
- Once it is off enough you should be able to remove the excess as follows: first, cut along the line where you want to the excess to come away; then lift the excess bead off as best you can in one piece; then go over the area where you have removed the excess with a meths-soaked rag to remove any residue.
- Get everything as clean as you want it to be using meths rags and elbow grease before the product has achieved 100% set.
Once you have finished:
- Lock the job away so that nobody will be tempted to go & poke thier fingers into it, you included - for 24 hours.
- Now, unless you have such a facility inside your firewall pass through the firewall removing your shoes as you go and then, without touching/brushing any handles, surfaces or switches, go into a well-lit, fully-tiled room equipped with a full-length mirror, strip to the buff turning your clothes inside out and putting them into a plastic bag as you go and then turn round slowly in front of the mirror looking for any little bits of Sika that you weren't aware of, cleaning them up as you go.
- When you think you are finished get a buddy to check you all over.
Do not be tempted to return through the firewall for a full 24 hours.