Is this the best stuff to get it clean again?

 

That and elbow grease.

 

First you need to check your boat is original gel coat and not been painted - if it's painted, ignore the rest of this post, it will probably remove the paint!
Then first stage is a straightforward wash - I would use whatever car shampoo you have to hand - that will probably get 90 percent of it. A pressure washer will also help.
Then use an oxalic acid product such as Y10 gel to clean all the stains - it will also work well on the green stuff - best thing about this product is that it doesn't use any elbow grease - paint it on and it does the cleaning as if by magic before you hose it off again a few minutes later. Don't be alarmed by the small size of the pot - one pot would do a 17ft boat at least twice over.
If you still have some really obstinate green bits, a bottom cleaner such as Starbrite boat bottom cleaner can be used - but I find Y10 gets it all.
Then once the boat is as clean as you can get it, the product you show comes into play - if you use before, you will just get shiny dirt! Personally I prefer the Farecla products - you need a cutting paste if the surface is really oxidised - but the crucial bit is to use a power polisher with a good quality sponge mop - you will never get anywhere near the same level of shine with elbow grease alone. If you get some really obstinate bits, you can wet sand with 1200 grit or finer wet and dry - I use washing up liquid to reduce its cut a bit more.
Then finally you have to seal the job if you're not going to have to do it all again in a short time - I use Autoglym Resin Polish.
Loads of how-to-videos on YouTube and I think on Salar's website there's a 'how to' guide as well.
Do the job well (be prepared to put some hours in) and you can get old GRP to look like the boat was built yesterday - the photo is a 12 year old boat.
For inside the hull, I like to use bilge cleaner - works really well on the grease etc that finds its way into the bilges.

 

I've been using the 3M stuff for years, it is really good and gives a lasting shine. When I bought "Osprey" I checked with the guys who had previously polished it and got some great advice on polishing. They recommended doing it by hand and NOT using a rotating buffer, it takes a while but I can do the hull of my 24 foot boat in a day. Pre cleaning much along the lines of Clinkers post.

 

They recommended doing it by hand and NOT using a rotating buffer, it takes a while but I can do the hull of my 24 foot boat in a day.

I wonder if that's because it had already been polished? - I've tried hand polishing v machine polishing and the difference between the results is night and day. I find it's the superstructure and cockpit of my boat that takes the time - machine polishing the hull takes me maybe 2-3 hours, the superstructure takes a lot longer.

 

ive always done mine as clinker above,and as said they come up as new.

 

Give a good wash then buy a product called Grunt. it's like a clear gel and will transform a dirty gel coat in half an hour without any scrubbing or rubbing. Wash off with water and hey presto, clean boat. Apply the polish and away you go. We have used it on dirty gel coat and rust stains etc and the whole thing comes up gleaming white. It's remarkable stuff. We gave the last of it to a couple in the boatyard that bought a yacht that was 30 years old. It went from a dirty buff brown to bright white in 30 minutes. Try it, you won't be disappointed .

Raider

 

Sounds the same as Y10 gel - any such product (oxalic acid suspended in gel) works brilliantly - I'd just go with whichever the chandler has in stock.

 

It's thirty quids worth of magic in a bottle. We had stains from rust,cuttle,bird poo and general grime over the winter. That stuff is amazing. Just paint the jelly on and starts working as you watch it. A couple of rust marks under the live bait tank from old hooks that had got under there from months ago just disappear in minutes. It turned our boat from a grubby cream colour into bright white in one morning. Check it out on their website. No bu!!sh!t it's brilliant.

 

It's thirty quids worth of magic in a bottle

£30!! Y-10 gel - enough to do my boat nearly twice - is around £10 and does everything you mention.

 

Bottle of oxalic acid, 6 russian migrants and a twenty pound note sounds like a winning formula!! Lol

 

Grunt is based on phosphoric acid, Y10 on oxalic acid.

At the risk of Salar calling me a miserable old git again.....

review and test of 10 competing products : http://www.pbo.co.uk/gear/10-gelcoat-cleaners-tested-26286

<bah humbug grumble>