What's the boat.
Year and storage conditions past and present.
Put up pics.

And why do you want too?

The biggest thing that causes issues, espec patches  including patches under paint etc is when stainless fittings are added (transducers lights etc) and are not insulated causing electrolysis.

After that is understanding that alloy corrodes/ oxidises (equivalent of steel rusting) very easy.. As it does so it, it forms an oxide. this oxide (unlike steel rust) has a very hard and attaches to the alloy very tight, with a molecule slightly larger than the alloy, creating a inprenatable barrier..
 What we see as a grey dulling.
 If the surface is highly polished, this also creates a surface that is resistant, but not permanent unless sealed (say a clear plastic or paint) It is also very prone to staining from glues, crc  and silicone lubes

or can you clean it well enough to bring back the shine?

 Deal with that 1st.

Yes.. remove excess oxidation with a weak acid. citric out of the missus spice rack. a spoon per 1/2L water , keep wet for and afternoon.  acetic acid or a very weak soln of phosphoric (rust kill).. the latter far faster and use with caution... test.

You may find using acid can be by past depending on how bad the surface is

 Then wet sand with about 380 grit till about 3/4 back to nice looking alloy..then with 800.  Depending on finish or going to cut with  polishing rouge then with 1200.

Now machine buff with a rouge ( I prefer stainless) and use kero as the medium and a cloth polishing on the polishing machine.. big angle grinder size.

Can you paint an alloy hull effectively

 yes.. important to degrease the surfaces BEFORE doing any sanding or polishing. Not doing so will 'grind any silcones/ waxes / oils into the alloy causing issues when painted.

Then as above sand back and finish around a 400 grit.. then send to a painter  who will use a paint system (from etch to finish coat) .. or can pay them to pre/ sand.
They use specialist high tech 2 pot car paint systems that require correct temps humidities to produce very hard and durable results


 

what is a rouge? I asked at the local engineering supplies store and drew a blank!!

Really???
They will have it on the shelf.
 Rough is  abrasive polishing compound made into a block like a hard block of soap.
Comes in many grades suitable for different metals and grades.. usually colour coded.. white being for stainless.
 Will be next to their cloth buffing wheels that go on bench grinders and drills.

 Wet sanding is using a wet and dry sand paper with a lubricant like water or kero... it stops the sand paper from clogging up...use a quality paper.. ends up being far cheaper in the long run.
 When driling or cutting .. even wet sanding  kero is the go.. but wet sanding you can go thru a lot..and generous water is near as good.

 When using rouge, kero is a more a luber/ spreader.
 Now hi polishing alloy is different to stainless, iron, steel , paint etc..
Those you are basically wearing the top surface off to get the final finish.
 Alloy you 'work it so you keep the area polishing quite hot .. not warm hot.. to the touch.
 Since alloy has a lower melting temp and is malable, what you are doing is getting big heat into the top few layers of molecules, these semi melt and fill in the deeper fine scratches.
Wear old clothes , polishing is a bloody messy job.

I have done many a V8 valley cover, holly and impco carbs, old Jowett bradford inlet manifolds that have laid in paddocks for 1/2 century+, mag wheels ,  and stainless flashings bumpers for 69 camaro ready to fit after a body rebuild and paint.
 And the fiberglass boat..
 oh and the glass on the shower door...
Over the last 50 odd yrs

Then wet sand with about 380 grit till about 3/4 back to nice looking alloy..then with 800.
 Will elaborate a little on that

 When sanding most ppl will sands till every mark is sanded out then proceed to the next, finer grit.
You do not have to...
 Get 80% the finer marks/ scatches out....which take say 3/4 of the deeper ones out.
 Then you go to the next finer to to get the scratches caused by the previous grit out.. this also makes the deeper ones even more shallow.
 And again next grit.. by this time the deeper scratches are so shallow they are starting to disappear.
 Then you cut with rouge and wheel... which as I describes above, sort of melts and fills these in.

Have i explained that ok?

Thanks for the explanations........is the abrasive polishing compound called "Rough" or "Rouge".......sorry, I'm a bit confused from the earlier posts!!

do you use a constant trickle of running water or merely wet the area at regular intervals??
 Constant trickle if on a concrete surface under foot...which dont mind getting messed up and stained.
 If on grass,  just dip and rinse/ shake in the water and sanding build up off the paper. And when lifting out carry generous amount of water to keep the surface wet enough to flow off excess sanding build up.
 Will not hurt to occasionally give surface a light rinse with the hose and rag.
 Once you get started you will very quickly get the feel for it nps
 

alumiprep

 this is a soln of phosphoric acid .. rust kill... fiber glass boat cleaner.. and a heap of other 'specialised' brand names... usually charging excessive prices

Very important when using any phosphoric acid product...rust kill whatever..
 Pour out what you need and use from that container.
 Do not use from the original container...
Any contaminates will react with the acid in the original container and neutralise it.
 Also its not going to burn your hands off...
I can dampen a rag and wipe down the bottom of my 5.5  F/ glass hull and skin gets a little dry for a while..
others maybe far more sensitive thu
DO NOT spill or drip it on you galvanised trailer.. it will instantly turn white and  eat the galv away.. not good.

The instructions above will take you to the piont of a mirror, and I mean mirror as in bathroom mirror finish.

You take it to which ever stage you wish...you may find a wet sand to 800 or 1200  will be what you are looking for...
 And on a small spot  do a etch with phosphoric to give a clean grey look.

Its more a time thing than hard work... unless out in the sun.

...to remove marks that appear to almost be etched onto the surface...

 These more 'deep' stains rather than scratches?

 Try a small spot with 150 and sand up...
Thu chances are they will be stains that have been etched into the alloy rather deep.

When getting into a resto project.. be it a muscle car, classic or even a boat...it is very easy to forgot you are working on a 2nd hand well used project, plus loose site of the end use.

 My 1st realization was on a muscle car that was intended to keep long term.. turned out 40 decades..totally dismantled in the shed..chassis all sand blasted back, and I was about to start grinding off and smooth out the factory welds.
M78 pointed this out at the time... do I want a trailer queen show car or something to put miles up on, drive in the rain, presents very well at a show and shine....

Rather than concentrate a lot of time on areas like this.. give a LITTLE extra elbow grease on each level of sand...and decide what you want to do towards the end of the build...
You may very well find everything gets knotched back a little and all good.

You dont need to turn out a show stopper, chrome plated see your reflection ,  finish..


 

Wind screen.. Meguires Quick detail..use as little as possble
 After a few 30 sec cleans on a screen that size, it builds up a protection.. wash the boat, stuff doesnt stick to the screen, dry screen with a towel to remove water mineral spots... 
Good on the clear shower screens to maintain as well.

Looks nice but a lot of coin....I like to think the natural alloy corroded look is cosmetic and thats all why paint it then you have to maintain?