Is the paste just a vehicle for keeping it on vertical surfaces? Oxalic acid will remove a lot of stains, but if your gelcoat
has truly turned to a yellow color and not just yellowed wax, imbedded dirt or other stain, I doubt oxalic acid will whiten it.
I just mix oxalic acid in water
in a spray bottle and spray it on. I use warmish water and stir in the OA until it doesn't dissolve anymore. Spray it on, wait until the color disappears and rinse it off. If it dries or takes more to get rid of the color - reapply.
It will damage aluminum
and tarnish bronze (actually clean it, but you will see it as discolored). No problem with stainless (you can use it to remove rust stains). I don't know about polycarb or perspex, but I wouldn't get it on them either. OA has some ability over time to make gelcoat porous if the gelcoat is going bad anyway. This will result in a dull look, but the gelcoat can be rebuffed and waxed after. It is always best to protect clean gelcoat as soon as possible, but it can wait a few days.
Best to wear gloves when applying and a particle mask when mixing and applying as a spray because OA isn't very friendly to humans. Inhalation shouldn't be a problem if you apply as a gel.
If you just have a stained waterline, "On And Off" is a product containing hydrochloric acid. It works like magic on waterline stains. It will also work on the whole boat, but is very detrimental to aluminum
and other parts
, which makes it difficult to apply well.