best material for a swim platform

The composite decking boards with wood fibre can absorb moisture over time as others have mentioned.

There are many pure plastic decking boards that are all recycled plastic which will not absorb water but tend to expand and contract quite a lot due to heat a trait that even a
wood/plastic composite board will do. In fact many of the composite boards use a hold down clip that permits movement to deal with the expansion.

No doubt aluminum is the lightest way to go and can be painted or powdercoated to match the boat

 

You can paint aluminum if it is properly prepped. Aluminum should be thoroughly cleaned and then etched with an acid to improve adhesion.
If you decide to use aluminum plate and want to finish it bright there is a product out there that works very well.

http://www.por15.com/Glisten-PC-High-Performance-Clear-Coat_ep_75.html

I haven't used it on my boat but I did restore a set of 4 alloy wheels on my 19 year old Suburban. They had become oxidized due to road salt that is heavily used in the northeast US. That was three years ago and my wheels still look great.

If you go the aluminum route and want a "bright" finish this product would be well worth using. It would be easier to apply to a flat aluminum plate than to a set of wheels.

Amazon sells it and you can see a review there that gives detailed instructions on how to prep and apply.

Good Luck,

MIA

 

"Plasteak" its the best. Supported by Stainless frame. Looks like wood, but no maintenance.