Gelcoat under rear bench seat was stained from rusted hardware (non SS staples and nuts & bolts) Apparently, water saturated the upholstery & padding
causing a lot of the wood bench seat to rot along with rusted metal stains on the gelcoat below.......looking for a quick fix ....Paint???....or can I just brush on
white gelcoat to cover?

I would try a strong solution if Iron Out and see what happens. I know I use it on our hull for the yellow stains

3pointstar

Have you tried a cleaner like Starbrite Rust stain remover?

Muratic acid, Sno Bowl or Works toilet bowl cleaner.

I tried muratic acid with a bristled brush last night although it cleaned surrounding top coat, it's pretty much the same as seen in pic w/deep seated rust stain
in gelcoat..........option now is to cover. My question is, can I just brush on white gelcoat to cover stain? -or- what type of white paint bonds good to gelcoat?

Try Whink Rust Stain Remover. It’s available at most local hardware stores and the big box stores too. I’ve been using it for super tough rust stains for over 25 years. Works on gelcoat, ceramic, porcelain, you name it. It’s hydrofluoric acid so it’s different than CLR and some of the others. All I can say is it has shocked me how quick and effective it is on countless ditch hope occasions kind of like yours. It’s only $5 too.

I would try on and off hull cleaner ,it's made by marykate. We keep it in stock , find it at most boat places. hf...

I would paint as last resort. These acid recommendations will work at some point