i am in the process of resealing my hull to deck joint on my formula 402. bolts are all out and old silicone removed. my question is should i use 3m 5200 or 4200? regular or fast cure? i am leaning to 5200 regular but was warned it will sag real bad before setting up. any advice is appreciated

Disclaimer... This is only my opinion.
I've actually been using Loc-Tite PL series (PLS30, PLS40) Polyurethane adhesive/sealants. They are a lot easier to work with than 5200 and come in black and white and 1/3 of the cost. They are outdoor home sealants. There is a bunch of individual marine testing on them. I used it on all of my thru hull fittings on the rebuild of my AT and plan to finish sealing the deck to hull on my Panther.

I know 5200 is near permanent, which sounds like what you want for the application. I don’t know how comparable other products are

you dont take a deck off a hull very often so 5200 will be there for life !

BUT !

I'd rather seal the hull to deck joint with something you can more easily remove or take off, using it as a water sealant, but tab deck to hull from inside with fiberglass.
This way you get peace of mind for security of deck staying on hull, but if ever removing the deck is needed, i feel cutting the tabing is easier that cutting the bond between deck and hull, and then split it open.

But that is just my opinion

I brought this up years ago and I believe Glassdave said to use 5200.

I did this across the stern and up each side a few feet. Went with 5200 in a caulking tube (actually 3 tubes). At the stern, the gap was close to 3/8" maybe a little more. Started pumping in the 5200 and abt 30 secs later it was running out. Could have been a disaster, but luckily I had some wide masking tape handy and was able to keep it in the joint. Came out pretty good. You will want to clean the surfaces the best you can and wipe with acetone so the 5200 adheres well. Can't remember if it was fast cure or not.

This method goes a lot smoother with 2 people. One shooting the sealant in and one following closely behind with the masking tape.

Lay some cardboard down on the floor in case it gets away from you.

One other thing I wish I had thought of. If you have a black rub-rail with exposed screws, use black 5200 on the screws.

There is a extensive thread under Formula section about the seam failures with videos and repair history and tips

IMO......Formula may have a safety isssue with the hull/deck bonding.

I've used the Loctite Marine and it doesn't sag and it bonds at least as good as 5200. I bought it at Home Depot in a caulk tube, perfect for your application. The deck should be a structural member and I would use the Loctite.

5200 is usually my "go to" for permanent bonding but ive also been known to use some of the lock-tite products you mentioned above. The one caution you need to be aware of is some of the non marine products use a hardener in the blend that does not play well with aluminum and can accelerate corrosion so if you choose to use them make sure its only fiberglass joints your sealing.