what should I use to mount an aluminum swim platform. I will through bolt it but what adheasive / sealent shoud I use 3m 4200 9r 5200?

5200 only time you will have to worry about it will be if you want to remove it then it will take a heat gun

4200 or 5200 will work as will silicone does well the important thing is good fasteners and correct sized holes so the fasteners don't work under laod, the 5200 can be gotten loose with DeBond a bit messy though.

Phil

I recommend 4200. It is safe for under waterline applications and will not remove th gel, fiberglass, wood, resin etc..if you need to pull the platform ever.

I would use the 4200, actually I did when I mounted my swim platform. same sealing as 5200, but about 50% of the adhesive properties.